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James78

Sealing hull before I paint it with an acrylic paint

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Hi. I've finished planking a Hull and I've put wood filler on it and sanded it down to a smooth finish. Just not sure about sealing it before painting. Was told to use an oil based polyurethane. Don't know much about painting, so do I just use a poly varnish? Can't get wipeon poly anywhere. It'll be painted later with acrylic paint. Any help appreciated. 

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Hello James,

No comment on your main question, more experienced builders will I am sure give good advice: however, if you are in the UK, see ...

... for my version of the hunt for water-based polyurethane varnish.

HTH

Bruce

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Yes, most poly finishes are oil based. Do avoid the water-based poly finishes. Wipe on poly is just thinned regular poly. You can thin yourself with mineral spirits or other suitable thinner. 2:1 poly to thinner would work, but you don’t have to be finicky about it. It’s thinned to flow on more smoothly and wipe on well. 

 

You don't actually have to “seal” the hull first before painting. The paint will seal it. But acrylic paint will stick fine to both the bare wood and wood sealed with poly. Thinned poly is probably better since you don’t want to build up much thickness in the finish. Acrylic paint doesn’t build a lot of thickness except after many coats. 

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Thank you both for your replies. It's my first ship to build and I've no clue about using polyurethane. I'll get some poly and mix it myself so. Just on the deck as I'm finished it too, does the poly give a nice stain or is there a load of different shades? And a bit of advice on sanding between coats please. I'm building the bluenose. I have a build log up on this site. Thanks again. 

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Sand the wood till smooth (400 grit for me). Remember, any imperfections will be much more visible after the paint goes on.

Sealing the wood will make painting easier. You can use a sanding sealer or use a primer. Modelling primers better but more expensive than automotive ones,

Polyurethane I 'd think would be the finish to use over the paint or on wood. I would have concerns using water based acrylics over oil based polyurethane. There is a rule, "fat over lean". But maybe others have had success. Never used polyuerethane myself.

 

Test first on scrap wood, not the model!

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That's what I was confused about

Using acrylic over oil. I don't know much about painting but I think I've read that somewhere before. But Griphos says it's ok, so I'm just puzzled now! Do I really need to use poly on the hull or would primer be enough? 

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Vaddoc is right, the old painter's rule is 'fat over lean'. However, if you sand down your sealing varnish, you would probably provide enough key for the hydrophilic acrylics to take a hold on the lipophilic varnish.

 

If you have already applied a sanding sealer, technically you won't need any other varnish, you can put the acrylic paint straight on. A coloured primer may have the advantage that you can see imperfections easier than on the wood. The primer should then also be acrylics-based.

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Thanks wefalck. I havnt done anything yet. I got a white primer with the paints. I have the hull really smooth and ready for painting when the time comes. I was just told to put a coat of polyurethane on it first to seal it before painting. Is it needed or will an acrylic primer do? I just don't want to mess it up. 

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As I said, when you used a sanding sealer, this should be enough to prevent fibres from raising, when applying a water-based paint.

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Paints, varnishes etc dry quickly but take a long time, many weeks, to fully cure and actually they can emit gases etc. This is were the fat over lean rule comes from.

Test on scrap wood. I actually did not about 2 months ago and went on and applied a dodgy varnish that I had concerns about directly over my rudder. Well, the whole think came out lumpy and horrible and then started peeling off. I had to carefully sand, re paint and re varnish the finished piece, taking care not to damage the water line and bottom half of the rudder. Painful experience!

 

Vaddoc

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A long time traditional first coat is shellac.  It is pretty much compatible with everything.  It is cut 1:1 with alcohol for the first coat.

As a final finish, it must be kept from water,  Water turns it white.

Many sand and sealer formulations are designed to be a primary base for open pore  wood species - such as Oak, Willow, Ash, Walnut.  It fills the pores and has a smooth surface for a final clear finish product.  These are generally thick and leave a layer that is out of scale on a model.  I prefer to avoid using wood species that have open pores - unless it is not visible. 

0000  steel wool between coats, leaves "tooth" for the next coat to bind with.  Just vacuum and tack rag to remove any steel fibers.  They will rust and stain.

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The main reason for the 'fat over lean' rule, comes from the old days, when painters used egg-tempera for roughing out the subject and finished their painting of in oils. It has something to do with the surface or wetting properties of the paints. Oil paints are 'lipophilic', which means 'grease-loving' and in consequence water-based paints, such as tempera or acrylics, will form droplets on their surface. They would not 'key' into the surface and also would not form the physicochemical (hydrogen) bonds needed to adhere to the surface. Oils or enamels on tempera or acrylics would spread out on the surface, rather than forming droplets due to the differen surface tension angles. Acrylics and tempera also do have a slight surface 'roughness' that lets the oils key in.

 

The sanding sealer I am using is essential a shellac with pumice suspended in. Rubbed down it provides a good base for the acrylics. 

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Guys just on the polyurethane. I can't get it in Ireland. The states seem to be the only place and it's expensive. I've seen videos of lads making their own with polyurethane and spirits. Went tí my local hardware and all they have is polyurethane varnish. Is this the same thing as just polyurethane? And why do people use this or other stuff to seal the hull? Can I not just sand it and prime it and then paint it? First timer so sry for all the questions. I just don't want to mess the paint job up. 

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1 hour ago, wefalck said:

Just use shellac. One should be able to get it everywhere. It's a traditional natural product and compatible with acrylics.

Thanks for that. 

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Posted (edited)

Shellac is an ideal sanding sealer. Hard to mess up. 

 

Polyurethane and polyurethane varnish are the same thing. 

 

You can can paint over properly applied poly just fine. Modern, good acrylics adhere very well to clean surfaces.  As a sanding sealer, you want to thin the poly by half at least and put on a very thin coat. You wipe it on and then wipe off the excess right away. It will penetrate the wood and not leave any real surface film. You do want to let it fully cure. A thin penetrating coat will cure in most climate conditions within 48 hours, but this is not guaranteed.  Fully cured, poly provides a hard durable seal.

 

I’m not sure why that is needed, though. Why are you wanting to “seal” the wood with a clear coat before painting?  Did you read that you should somewhere?  As you say, you can just sand and paint.  Wipe the sanded surface with mineral spirits. A tack cloth will not get all the dust. 

 

Again, the paint itself can be used as a primer/sealer. I don’t seal surfaces I paint, and don’t usually use primers. A thin first coat does the job. Always build paint in thin coats. Don’t try to make it opaque in one go. Multiple thin coats of a good acrylic, like Vallejo or Polly Scale, will leave a beautiful and very durable finish. 

 

If you want to seal first, shellac is best. Most hardware stores sell a “sanding sealer” that is shellac. Most sell Bullseye here in the States, which is a decent pre-made shellac. I always use blonde or super blonde flakes and make the cut I want myself with denatured alcohol.  But I usually use shellac as a clear finish for natural wood, not as a sealer.  Blonde if I don’t want to darken the wood tone, and darker flakes if I do. But several coats of shellac will leave a high sheen. That’s where a wipe on poly can be nice. Poly can be obtained with additives that dull the sheen to a satin finish, which works well for decks, etc. 

 

I’d just sand and wipe with mineral spirits and start applying thin coats of paint. But I suggest that you make a sample of some planks glued together and test your paint and technique with that first. Don’t start painting the hull until you can paint a sample in a way that looks very good to you. What paint are you going to use?  That’s the more important question not all paint is equal. 

 

 

Edited by Griphos

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1 hour ago, Griphos said:

Shellac is an ideal sanding sealer. Hard to mess up. 

 

Polyurethane and polyurethane varnish are the same thing. 

 

You can can paint over properly applied poly just fine. Modern, good acrylics adhere very well to clean surfaces.  As a sanding sealer, you want to thin the poly by half at least and put on a very thin coat. You wipe it on and then wipe off the excess right away. It will penetrate the wood and not leave any real surface film. You do want to let it fully cure. A thin penetrating coat will cure in most climate conditions within 48 hours, but this is not guaranteed.  Fully cured, poly provides a hard durable seal.

 

I’m not sure why that is needed, though. Why are you wanting to “seal” the wood with a clear coat before painting?  Did you read that you should somewhere?  As you say, you can just sand and paint.  Wipe the sanded surface with mineral spirits. A tack cloth will not get all the dust. 

 

Again, the paint itself can be used as a primer/sealer. I don’t seal surfaces I paint, and don’t usually use primers. A thin first coat does the job. Always build paint in thin coats. Don’t try to make it opaque in one go. Multiple thin coats of a good acrylic, like Vallejo or Polly Scale, will leave a beautiful and very durable finish. 

 

If you want to seal first, shellac is best. Most hardware stores sell a “sanding sealer” that is shellac. Most sell Bullseye here in the States, which is a decent pre-made shellac. I always use blonde or super blonde flakes and make the cut I want myself with denatured alcohol.  But I usually use shellac as a clear finish for natural wood, not as a sealer.  Blonde if I don’t want to darken the wood tone, and darker flakes if I do. But several coats of shellac will leave a high sheen. That’s where a wipe on poly can be nice. Poly can be obtained with additives that dull the sheen to a satin finish, which works well for decks, etc. 

 

I’d just sand and wipe with mineral spirits and start applying thin coats of paint. But I suggest that you make a sample of some planks glued together and test your paint and technique with that first. Don’t start painting the hull until you can paint a sample in a way that looks very good to you. What paint are you going to use?  That’s the more important question not all paint is equal. 

 

 

Thanks for your reply. I was advised to seal it with poly. Got myself in a twist then looking up different sealers and using acrylic over poly. Everyone seems to be right but no two answers are the same so I just got confused. The paint I'm using or thinking of using is the paint I got from model shipways. I bought the paint set to suit the bluenose. 

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8 minutes ago, James78 said:

Thanks for your reply. I was advised to seal it with poly. Got myself in a twist then looking up different sealers and using acrylic over poly. Everyone seems to be right but no two answers are the same so I just got confused. The paint I'm using or thinking of using is the paint I got from model shipways. I bought the paint set to suit the bluenose. 

As u can see, the hull is almost all covered with fillers. Don't know if that makes any difference? 

IMG_20190602_172307.jpg

IMG_20190602_172245.jpg

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Just put a coat of shellac on it and then sand with very fine grit sandpaper until it's as smooth as a baby's bottom. If you sand through the shellac, which shouldn't happen with a 300 grit or less abrasive, reapply the shellac where it's sanded through and sand again until it's perfectly smooth. Shellac  comes in different "cuts," and is marketed as, for example, "two pound cut." That would mean two pounds of shellac flakes dissolved in one gallon of alcohol. Shellac can be the consistency of molasses and used as an adhesive in some applications, or as thin as water (2 pound cut) for use as a sealer. If it thickens in the can, just add alcohol and stir well. Don't worry about how much you have to thin it. Just get it to "water" consistency and you're good to go.

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Use of a primer with paint probably comes from full size practice.  The first coat soaks in, uses a lot of paint,  and leaves an incomplete surface.  A primer  came have a budget level cost and finish paint can be expensive.  For large surfaces and a lot of paint, it saves money by using a primer.

If you have a surplus of finish color, a separate primer has no savings advantage, so using the finish as primer will work.

If the surface has a mold invasion, or it is a wood species with sap or rosin, a proper sealing primer is usually wise, it keeps the mold or sap from migrating to the surface over time.

 

I wonder if the advise to use poly as a primer came from a source who thought a way was found to show up the traditional suggestions made by professionals in wood finishing.  Polyurethane is a synthetic plastic,  if you are not a moldy fig about using "modern" materials on a model, it seems to have been shown to have stability over time.

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Thanks very much lads. I think I just over think things. I was the same with the planking. I ended up doing a good job in the end. I think I'll just go with the shellac to seal it and prime and paint it then. Not used to painting at all. If I'm to do several light coats, what grit sandpaper will I do between coats? Thanks again. 

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Just to note: primers and sanding sealers are two different animals. I pays to reflect upon why one uses what and what the function respectively is.

 

Primers have the function to act as intermediate between the surface and the chosen paint. The surface and the paint may not be compatible from a physicochemical or surface chemistry point of view. For instance, brass or copper are slightly water repellant due to an oxide layer forming on them quickly. Acrylics would not stick on these metals very well. Hence, a primer that reacts with the surface and leaves behind a new surface that is compatible with the acrylic paint.

 

A sanding sealer (including 'filled' shellac) has the purpose to fill the pores of wood, harden the wood and thus to aid in sanding. Traditionally, furniture surfaces are build up from multiple applications of shellac of dfferent dilution, moving from pumice-filled to pure shellac. If you will be using a paint that dries as hard as e.g. shellac, you may forego the sanding sealer and build up the surface with that paint. Modern acrylics, however, can take month or even a year to reach a state of polymerisation and hardness that allows you to sand in the same efficient way as shellac. So, applying a sanding sealer first and then the paint is a more efficient process.

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Posted (edited)

You may have been advised to use poly, or some sealer, in order to “harden” and seal the filler, since there does appear to be more filler than wood that is exposed. That’s not bad advice, depending on what filler you used. Some fillers harden well. Some stay more grainy and it is possible to scratch or dent them more easily than the wood.

 

Shellac can provide a hardened surface to the filler to apply paint to. It was used to create the hard shell on M&M candy originally, which is why they would not “melt in your hand” (I have no idea if it is still what is used). It’s totally natural and fully edible. The person probably suggested poly because it may soak into the filler to some degree (since it is an oil) and harden, providing a better or more durable surface to paint and handle.  Shellac will provide a surface shell as well, but won’t soak in. And a single thin coat won’t build much of a shell. Enough for paint to adhere well, however. And multiple layers of paint will be both flexible and protective. I’ve not used MS paints, and don’t know who makes them, so can’t advise about them. I’m sure others here have. 

 

Sorry if all all of this is confusing. Totally understandable. Finishing is a complex matter, and although there are basic fundamentals, there’s no easy one-step fits all situations process. But at our scale with our materials, it tends to usually work out just fine rather than not. 

 

The key factor in your situation is the filler. I think most of us have been giving advice under the assumption that you are wanting to seal natural wood. Some fillers are made to be painted and can be painted directly with good results. In a sense, such fillers perform both a sealing and priming role already. 

 

What filler did you use?  Is it a hard surface?  Does handling it create any dust or leave residue on your hands?  You’ll want to make sure it’s clean before any finish. So a light sanding with 320 or 400 grit paper will present a fresh, clean surface for the finish to adhere well. I don’t know that you should use mineral spirits to clean, since that may react with the filler and perhaps soften it.

 

I'd still suggest making a few samples of glued planks or some solid sheet and apply your filler on top and let it fully harden and dry. Then apply shellac to one, and paint; prime another and paint, and just paint the third.  You’ll see what gives the best result in your situation that way. The likelihood is that all three will be very similar.

 

As for building multiple thin coats of the model paint, you shouldn’t need to sand between coats, particularly if you thin the paint so it flows well, or it comes that way, like Vallejo and Polly Scale paints. If you are airbrushing, and have thinned the paint enough to flow well through the airbrush, then no sanding will be necessary. Acrylic paint dries fast, particularly through an airbrush, and that brand may need retarder added so that it airbrushes well. Some brands don’t.  I’d just use brushes to paint the hull myself.  

 

 

Edited by Griphos

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56 minutes ago, Griphos said:

You may have been advised to use poly, or some sealer, in order to “harden” and seal the filler, since there does appear to be more filler than wood that is exposed. That’s not bad advice, depending on what filler you used. Some fillers harden well. Some stay more grainy and it is possible to scratch or dent them more easily than the wood.

 

Shellac can provide a hardened surface to the filler to apply paint to. It was used to create the hard shell on M&M candy originally, which is why they would not “melt in your hand” (I have no idea if it is still what is used). It’s totally natural and fully edible. The person probably suggested poly because it may soak into the filler to some degree (since it is an oil) and harden, providing a better or more durable surface to paint and handle.  Shellac will provide a surface shell as well, but won’t soak in. And a single thin coat won’t build much of a shell. Enough for paint to adhere well, however. And multiple layers of paint will be both flexible and protective. I’ve not used MS paints, and don’t know who makes them, so can’t advise about them. I’m sure others here have. 

 

Sorry if all all of this is confusing. Totally understandable. Finishing is a complex matter, and although there are basic fundamentals, there’s no easy one-step fits all situations process. But at our scale with our materials, it tends to usually work out just fine rather than not. 

 

The key factor in your situation is the filler. I think most of us have been giving advice under the assumption that you are wanting to seal natural wood. Some fillers are made to be painted and can be painted directly with good results. In a sense, such fillers perform both a sealing and priming role already. 

 

What filler did you use?  Is it a hard surface?  Does handling it create any dust or leave residue on your hands?  You’ll want to make sure it’s clean before any finish. So a light sanding with 320 or 400 grit paper will present a fresh, clean surface for the finish to adhere well. I don’t know that you should use mineral spirits to clean, since that may react with the filler and perhaps soften it.

 

I'd still suggest making a few samples of glued planks or some solid sheet and apply your filler on top and let it fully harden and dry. Then apply shellac to one, and paint; prime another and paint, and just paint the third.  You’ll see what gives the best result in your situation that way. The likelihood is that all three will be very similar.

 

As for building multiple thin coats of the model paint, you shouldn’t need to sand between coats, particularly if you thin the paint so it flows well, or it comes that way, like Vallejo and Polly Scale paints. If you are airbrushing, and have thinned the paint enough to flow well through the airbrush, then no sanding will be necessary. Acrylic paint dries fast, particularly through an airbrush, and that brand may need retarder added so that it airbrushes well. Some brands don’t.  I’d just use brushes to paint the hull myself.  

 

 

Thanks you again for a great reply. The filler I used is ronseal multipurpose wood filler for general wood repairs. 

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I'm not familiar with that brand.  A quick google and it appears to remain flexible (doesn't shrink or crack), so it may have some acrylic or other flex agent in it already.  It does say you can nail into it, so I suppose it hardens somewhat (although I doubt that it will hold a nail for any length of time).  At any rate, it also says you can paint directly on it.  And it supposedly takes varnish as well.  

 

I suspect you have a fairly thin layer of it over most of the surface.  I'd still try the sample pieces first, since we don't really know what sort of material the filler is, or its chemical makeup, and see how it responds to the various treatments.  

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3 minutes ago, Griphos said:

I'm not familiar with that brand.  A quick google and it appears to remain flexible (doesn't shrink or crack), so it may have some acrylic or other flex agent in it already.  It does say you can nail into it, so I suppose it hardens somewhat (although I doubt that it will hold a nail for any length of time).  At any rate, it also says you can paint directly on it.  And it supposedly takes varnish as well.  

 

I suspect you have a fairly thin layer of it over most of the surface.  I'd still try the sample pieces first, since we don't really know what sort of material the filler is, or its chemical makeup, and see how it responds to the various treatments.  

It's very hard and shiny too after sanding as u can see in picture attached. Not sure if all fillers look that way. I literally covered the whole hull and sanded till I got it smooth so it's thin in places but thicker in other areas. 

IMG_20190522_133346.jpg

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Hard is what matters, and stable (doesn't crumble or dust).  One nice thing about using a primer is that both primer and paint will telegraph the imperfections well.  Even small scratches you don't see will show up when primed/painted.  A sanding sealer tends to hide imperfections (partly due to the sheen).  So, it might be a good idea to prime it just to spot the problem areas and sand them smooth.  You want the smoothest surface possible before painting.  For both clear coats and paint, time spent prepping the surface pays dividends.  

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There is a lot of experience here...do some research, then you will have knowledge now and for the future! Experiment on scrap and you will judge for yourself. Nice work so far, finish it right.

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I used trade wood filler once, the equivalent of ronseal. It was a disaster, it was rock hard, impossible to sand, risked destroying the hull. I now use only Elmers which is ideal for modelling

 

 https://www.amazon.co.uk/Elmers-X-Acto-16oz-White-Multicoloured-9-65x9-65x10-79/dp/B00JKB2GP2?SubscriptionId=AKIAILSHYYTFIVPWUY6Q&tag=duc08-21&linkCode=xm2&camp=2025&creative=165953&creativeASIN=B00JKB2GP2

 

Many ways to skin a cat. Test on scrap wood, learn how the various products behave and see what works for you. 

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30 minutes ago, vaddoc said:

I used trade wood filler once, the equivalent of ronseal. It was a disaster, it was rock hard, impossible to sand, risked destroying the hull. I now use only Elmers which is ideal for modelling

 

 https://www.amazon.co.uk/Elmers-X-Acto-16oz-White-Multicoloured-9-65x9-65x10-79/dp/B00JKB2GP2?SubscriptionId=AKIAILSHYYTFIVPWUY6Q&tag=duc08-21&linkCode=xm2&camp=2025&creative=165953&creativeASIN=B00JKB2GP2

 

Many ways to skin a cat. Test on scrap wood, learn how the various products behave and see what works for you. 

It is rock hard but the hull turned out nice. I'd use it again as I found sanding it with 80grit did the job. Products like Elmers and minwax only to be got in the states which is a balls but I'm sure other products do much the same job. It is my first ship kit so it's all a learning curve for me. Got great support from this forum so thanks everyone 👍👍

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