Sanding Sealer - when/how to use it

[fifthace]

Hello.

Forgive me if this question has already been asked a 1000 times, I've searched and searched and I am still nonethewiser.

Sanding Sealer.

When to use it and how to use it.

For example, I have a build I am planning where the hull will be a mix of painted below the waterline, natural wood (stained?) between the water line and upper section, then painted for the bullwarks.

I've read one comment that says you can't stain after sanding sealer and another that says you can?
Do you use sanding sealer before or after sanding?
Do you use it before paint or after?

And how does it differ from varnish?


Thanks in advance.

[Dave_E]

Hi Dominic,

 

My experience building guitars is stain first…. the stain soaks into the wood, that’s what gives you the uniform new color. Then seal (if you want to). On a model ship, I guess you could stain the entire hull, then seal it, then paint it. The sealing prevents your paint from getting soaked up by the wood. Paint is the last step… always. I just did my first painting on them Lady Nelson… painted the inside of the bulwarks and it cook 2 coats because the wood soaked up the first coat. 
 

To seal or not to seal. What kind of final finish are you after?

Edited February 18, 2022 by Dave_E

[fifthace]

Hello Dave,

Thanks for a quick response. This is the build I had in mind. I did this about 5 years or so ago, but it got destroyed.

I'm taking the opportunity to redo this, fixing the mistakes I made with this one. So it will be pretty much the same except I won't be coppering the hull, instead it will be white.

Overall I think I'd probably want a satin finish. I don't like high gloss, I always feel it makes things look too toy like, but at the same time I don't want a completely flat finish either, so satin is probably the one I want.

I do remember painting this was a complete pain as I didn't seal first and it took far too many coats to get colour and coverage, and I want to avoid making that mistake this time.

Thanks

[Dave_E]

VERY NICE!  The ocre colors are all pretty dull in my experience. Sealer will/might fill the pores in the wood. When those pores are filled, you might loose the real wood look. I am going to fill below the waterline, then paint. On smaller tight areas, trying to seal wood might get sporty as it should be sanded before paint. I suppose someone could seal bare wood before assembly. 
 

I have read many posts about coating everything with varnishes, shellac, sealers etc.. I’m kind of a build the wooden ship, it looks like wood, feels like wood and put it in a case for protection against the elements kind of guy. 😊

[Jaager]

These terms are often used in a fast and loose manner with no anchor to what they really mean, so some posts can be confusing.

 

I think these are functionally precise definitions:

 

A sanding sealer, or sand-n-sealer -  a clear and thick mixture with micro particles -  the main use is as a base coat on furniture built using open pore wood species such as Oak, Walnut, Ash...   It fills the open pores so that when viewed at an oblique angle, the final surface appears to be glass smooth.

Opinion:  it is too thick for scale use.  Open pore wood is best not used at all on scale models if the wood is to be clear finished.  There are other ways to fill pores before painting.

 

A primer is generally a 50% diluted clear material intended to soak in deeply and limit any additional material layers to being surface only.  The traditional primer is half strength shellac.  It is easy to apply, easy to undo, quick to dry, low cost, and compatible with most any other materials applied over it.

If an clear oil finish is the goal, half strength Tung oil can be its primer coat or I am guessing half strength Linseed oil will serve as as its primer.  Both would want additional time to polymerize before being covered over.  50% shellac is probably more cost effective as a primer for these. 

Shellac is alcohol based and the oils are organic solvent based.  They do not raise wood grain.

Paint can be its own primer, it just requires more coats to get a dense enough surface layer.  If the paint is water based, the surface may need fine sanding to remove any raised grain.

 

A dye is a monomolecular solution of a pigment that soaks into wood and changes its color.  It does not change the surface.  It does not obscure wood grain.  If anything, it increases the contrast,  This is not necessarily a wonderful thing on a scale model, so having grain with lower contrast is a deciding factor for the choice of wood species if it is to be clear finished.

Dyes come water based and alcohol based.  The water based dye soaks in more deeply, but the first application raises wood grain unless a dilute PVA/water solution is used first and then sanded when dry.  The alcohol based dye does not raise grain and on a model, any effect that a deeper water dye penetration may provide is probably too slight to be noticed, so alcohol based is probably the better choice.   No primer effect with either is there.

 

A stain is a suspension of pigment particles in a solution with a polymerizing binder.  This is also the definition of paint.  The pigments are wood colored and some commercial products may also contain a dye so that it can advertise that it penetrates.  

Opinion:  on a model, if it is wood that needs a stain before clear finish it would serve you better to replace this wood species with one that needs no pore filling or grain hiding with a semi-transparent paint - which is what a stain is.  The purpose of a stain is to try to make a low quality wood appear to be a high quality wood ona piece of economy furniture.

[fifthace]

Thanks for this, I will keep this to refer to.

Thanks for taking the time to write all of that.

[wefalck]

As a matter of fact the transition between varnish, sanding sealer and primer are rather flowing. 

 

A primer is a material that interacts physico-chemically or chemically with the surface to which it is applied and thus adheres well to it. In turn, the paint interacts with the primer and therefore adheres well to it. With the exception of certain oily wood species, there is normally no need to prime wood for painting.

 

A sanding sealer essentially is a kind of varnish in which finely ground particles, often pumice powder are suspended. The idea is that the varnish penetrates into the wood and the particles fill up any surface structure and pores of the wood. There are many commercial formulation of sanding sealers available. While they were originally based on solvents, today acrylics based formulations with water as solvent are more and more sold for environmental protection reasons, though they do not penetrate so well, particularly into more resinous wood species. Sanding sealer is applied after a first sanding that has been rubbed down wet to raise the wood fibres. It is then sanded down in preparation for further steps. Sometimes a second application is needed. Rather than sanding, I prefer to either scrape (flat surfaces) or rub down with steel-wool (curved surfaces).

 

A dye or stain is a coloured organic molecule dissolved in water, alcohol or an organic solvent. This means a dye can only applied to material that is capable to soak up the solvent you are using. It implies that you have to apply any dye or stain after a first sanding, but before applying sanding sealer.

 

An ink, conversely, is a liquid, usually water or alcohl, in which finely ground solid particles (e.g. soot) or coloured organic partciles are suspended. The ink also contains a binder, that may be either water soluble or not - in the latter case it is referred to as permanent ink.

 

A varnish is a lacquer dissolved in an appropriate solvent. A classical example is shellac, which is dissolved in alcohol, which gives it a good penetration into wood. Other lacquers and solvents may be used in commercial formulations, such as acrylic or polyurethane varnishes. The varnish would be applied to the surfaces prepared with a sanding sealer. One has to pay attention that the solvent of the varnish does not attach the sealer, otherwise you may destroy the effect of sanding.

 

A paint essentially is a varnish that has a pigment added to it. Again, paint is applied on top of the sanding sealer and the same provisa as for the plain varnish applies, i.e. sealer and paint have to be compatible. In general, water-based acrylics are compatible with almost anything, as water is not a very agressive solvent.