PVA glue as a sealant for wood

[DaveBaxt]

I recently joined a local modelling club where the people are all into plastic modelling of all different types. I am currently putting together the 18lb Cannon carriages for my next build . Although I am intending to use diluted shelac or similar to seal the pear wood prior to painting. I thought about airbrushing the medium  for sealing if possible. Of coarse the guys at the club were very helpful and they all agreed that diluted PVA glue would be the answer..

                Although this initially sounds like a good enough idea, ie. Cheap and readily available, I then wondered to myself . Why is pva not extensively used by the people on this forum for the above purpose and wondered if it is due to raising the grain or something else which I am unaware of. I just thought this would be a good subject to discuss as I believe that the there are a lot of knowledgable people on this forum especially on the subject of all things wood and how to seal it . Especially using an airbrush and, is this is even possible. As there is some very intricate detail on the parts of the carriage that I intend to paint, I do not want to lose them by filling up the indentations with sealant. Best regards Dave

[wefalck]

The point of a wood sealer or sanding sealer is that it should penetrate into the wood. There are two properties that help this process, low viscosity and good wetting behaviour. These are properties that PVA doesn't really have, it has, if undiluted, a high viscosity and being 'polar' (sorry for the chemistry-speak), it is being repelled from more or less resinous wood. Good sanding sealers are based on some kind of organic solvent (which includes alcohol in the case of shellac) and quite thin, so it really soaks into the wood, rather than forming a layer on top.

 

A sealer has to be 'worked' into the wood - in the old days shellac was applied with a 'tampon'. Modern sanding sealers are usually applied with a bristle-brush. So, application by air-brush would be counterproductive.

Edited February 17, 2023 by wefalck

[Jaager]

A primer of half saturated shellac is easily applied using a cotton rag - a worn out T shirt or bed sheet - it soaks into the wood.  Using the rag  - no ponds are left on the surface - it does not hide detail.

Indeed, if you had used an open pore wood species like Oak or Walnut, the open pores would still be there.  The application is less involved than using an air brush.  It is quick and simple to use.  It takes many applications of full strength to build any sort of thickness - an application is just a wet layer, not a coat.

Use scrap wood and practice a few times - you will then realize that it is not as complicated as imagined.  You can buff with a Scotch Brite pad when dry - 0000 steel wool is traditional, but the steel shards - not seen, but there - make themselves known when they turn to rust and leave a stain.

[Bob Cleek]

What Eberhard and Jaager said! Absolutely. Canned pre-mixed shellac is readily available and inexpensive (although I've heard tell that it is harder to find in England for some unknown reason, so you may have to buy dry shellac flakes and mix it up in alcohol yourself.) Denatured alcohol, with which it is thinned, if necessary, and for clean-up, is also readily available and inexpensive. (Use the forum search engine for "sealing" and "shellac" and you'll find lots of details on its use as a sealer.) Thinned shellac (what they call two or three pound "cut," a mix ratio of two or three pounds of flake to a gallon of alcohol, (you'll need much less than that mixed at a time for modeling use) is the consistency of water and soaks right into the wood without raising the grain. I often simply dip a part into the can and let the excess drip off back into the can. It dries very quickly. I have no problem with losing detail at all. Were there a problem with the thickness of the application, the excess shellac, even if dried, could easily be wiped off with a cloth soaked in alcohol. Dried shellac-sealed wood can be rubbed or sanded easily. Any oil- or water-based paint can be applied over dried shellac. I really don't think there is a better wood sealer for modeling purposes, all things considered. Others prefer using other types of commercial sealers, but nobody I know uses diluted PVA for the purpose.

Edited February 17, 2023 by Bob Cleek

[DaveBaxt]

Thanks guys for the information and what has confirmed that I already thought that there are better products out there than pva glue. I am currently using a product called Bulls eye Seal coat which I am informed is Shelac,at least that is what it says on the Tin. Interestingtly enough it does not go off or at least it hasn,t yet ,when I left the top off a small bottle overnight. I like the sound of Bobs idea of dipping small parts into it then easily wiping off any excess. I have some ethenol and methelated spirits without the purple stuff so I  can thin it down so will quickly run out of areas,if that makes sense. Anyway I have also used this diluted on rigging knots and lines and I believe it to be a stronger hold. 

          Thanks again guys for help and patience . You will make a modeller of me yet. At least I hope you will. best regards Dave

[wefalck]

Shellac is also a traditional cement used by jewellers and watchmakers. The latter use it in molten form to stick (temporarily) metal parts together. Dipped in alcohol, the parts separate.

 

A while ago, I got myself shellac-flakes and dissolved a lot of them just in a few millilitres of alcohol to make a cement with honey-like consistency. It grabs quite well, but it takes quite a while for the alcohol to evaporate. When working with it, the 'open' time is quite infinite, giving you lots of time to adjust parts. As any excess can be wiped off with a brush dipped into alcohol, it makes for a neat cement.

[DaveBaxt]

1 hour ago, wefalck said:

Shellac is also a traditional cement used by jewellers and watchmakers. The latter use it in molten form to stick (temporarily) metal parts together. Dipped in alcohol, the parts separate.

 

A while ago, I got myself shellac-flakes and dissolved a lot of them just in a few millilitres of alcohol to make a cement with honey-like consistency. It grabs quite well, but it takes quite a while for the alcohol to evaporate. When working with it, the 'open' time is quite infinite, giving you lots of time to adjust parts. As any excess can be wiped off with a brush dipped into alcohol, it makes for a neat cement.

 

 

Sounds like something we could use instead of super glue or 2 part epoxy resin but then again perhaps it does not have the strength of epoxy. Good to know all the same. Thanks again for your input. Best regards dave

1 hour ago, wefalck said:

    

 

[Roger Pellett]

I am reading Rob Napier’s new book Caring for Ship Models, published by Seawatch Books.  For readers not familiar with Mr Napier, he is a professional ship model restorer, hired by major institutions and serious ship model collectors.  No, he doesn’t work on that masterpiece that you built from a kit bought on on EBay.  🤣

 

In the book, he describes restoring the cracked and chipped shellac finish on the figurehead of a very old Dockyard style model.  It was not necessary, or desirable to remove the old finish or to brush on new shellac.  With a small brush and alcohol solvent he was able to dissolve and redistribute the old shellac.

 

Roger

Edited February 18, 2023 by Roger Pellett

[Bob Cleek]

2 hours ago, Roger Pellett said:

It was not necessary, or desirable to remove the old finish or to brush on new shellac.  With a small brush and alcohol solvent he was able to dissolve and redistribute the old shellac.

Ditto for refinishing pianos and other aged French-polished furniture.

[vaddoc]

Good advice above, however I personally never liked Shellac or PVA for sealing wood. I ve been using Decoart wood sealer for years. Water based, dries in 10 min. Absolutely wonderful stuff. A bottle will last years and years - shelf life it seems indefinite.

 

Deco Art Americana Multi-Purpose Sealer-8oz, Other, Multicoloured, 236 ml (Pack of 1) : Amazon.co.uk: Home & Kitchen

[wefalck]

Would be interesting to read the technical information sheet for this. From the description I would guess that is is a pure acrylics solution, no pigments and fillers. In this sense it is a primer, rather than a sanding sealer.

 

How does this stuff sand? My experience is that acrylics are too soft to sand well or at least one has to let them cure for weeks.

[vaddoc]

It works wonderfully. It is not a primer, it is a sanding sealer. Water based so no toxic fumes. Dries in 10-15 min. Sands beautifully. Leaves smooth sealed wood. I have no need to use anything else.

Usually slap a coat on sanded wood with no real care, wait 15min, slap another coat with or without a quick 400 grit sand, wait another 15 min, then do final 320 or 400 grit or whatever and wood is sealed and smooth. Completely colourless. Job done!

Edited February 19, 2023 by vaddoc

[Bob Cleek]

1 hour ago, vaddoc said:

Water based so no toxic fumes.

Shellac's solvent is alcohol. I'm living proof that alcohol fumes are not toxic.    

[vaddoc]

9 hours ago, Bob Cleek said:

Shellac's solvent is alcohol. I'm living proof that alcohol fumes are not toxic.    

Indeed Bob!😁

I was referring to methylated spirits.