Shellac as a Sealer before Acrylic Paint?

[turangi]

I recently sealed some wood with Shellac as a sealer prior to brushing Tamyia acrylic  paint as the top coat. While brushing on the paint the whole finish turned into a sludgy mess. I had let the Shellac dry for several days but wonder if that since the Shellac and Tamyia paint both contain Alcohol as a solvent that the Alcohol in the paint caused the Shellac to re-emulsify? Has anyone else encountered this problem?

[Dr PR]

Instructions for using shellac on furniture caution against getting alcohol or water on the finished surface. Water can cause it to turn a lighter color. However, they say that you can apply alcohol with cotton sponges and reflow the shellac to restore the finish.

 

It is a good bet that the alcohol in the paint softened or dissolved the shellac.

[turangi]

29 minutes ago, Dr PR said:

Instructions for using shellac on furniture caution against getting alcohol or water on the finished surface. Water can cause it to turn a lighter color. However, they say that you can apply alcohol with cotton sponges and reflow the shellac to restore the finish.

 

It is a good bet that the alcohol in the paint softened or dissolved the shellac.

Exactly my thought but I see it recommended here often as a sealer compatible with almost any finish material.

[wefalck]

Yes and no - as always. There are acrylics and acrylics. I have no practical experience with Tamiya paints, but indeed understand that they contain inter alia alcohol as solvent. I know from experience with pieces of furniture that I built that shellac finishes turn milky when you put a wet cup or, worse, a glass will alcohol on the outside on it.

 

The problem, however, is not necessarily the water or alcohol itself, but the way how the paint is applied. When brushing on paint, you bring a lot of solvent onto the surface that then has a lot of time to interact with the surface, while it is evaporating. Airbrushing here is the solution.

 

Also, when used as a sealer, the shellac should be rubbed down, e.g. with steel wool, so that it does not form a layer on the wood anymore. Then, even brush-painting should have less effect on the sealer. Several thin coats of paint are also less likely to interact with the shellac, as the little solvent from the first coat will evaporate quickly and then protect the shellac.

Edited November 12 by wefalck

[turangi]

Thanks for the replies! I actually tried Tamyia fine primer today on wood and it worked great under Acrylic paint even though listed for plastic and metal. No grain raising and didn’t obscure details.

Edited November 13 by turangi

[rcweir]

Alcohol will dissolve shellac.   That rule always applies, no matter how long the shellac has been down.    But another rule (for furniture finishes) is that any finish can be applied on top of a wash coat of shellac.    I'd suspect in this case that the shellac mixture was too thick; I use a 1/2 pound cut for wash coats, which means the shellac was mixed in the ratio of 1/2 pound of shellac flakes for each gallon of alcohol.   That's contrasted with top coat cuts of 2-4 pounds or more.   I don't know what pre-mixed cuts are like,  but I would think they're much thicker than 1/2 pounds.    The main reason for the wash coat (again, this is me speaking and not a finishing oracle) is a transition step between sanding and applying the finish.  The wash coat will raise the grain, which can be sanded down with a fine grit, and then the top coat application will have a much reduced effect on the wood's smoothness.   

 

I would not expect that the wash coat of shellac is going to be any help unless one expects to lightly sand after it.

 

(We were also taught that "sealing the wood" was impossible.   Discussions along those lines instead used terms like "filling the pores" and "leveling the surface", because one goal for the finisher is a desired final surface quality.)

[wefalck]

I think the term 'sanding-sealer' doesn't really mean sealing the wood hermetically. It at all this could only be achieved with a glazing that is impermeable to water vapour.

 

Also, I find the term 'finish' confusing or misleading, as it can mean different things, ranging from the material used to the degree of sheen.

 

In classical French polishing one tries to achieve basically two things: the depth of colour that comes from the light penetrating into the pores of the wood and not being reflected from the surface, as described above, and a smooth, possibly glass-like and shine surface. Traditionally, this was achieved by rubbing the wood with blocks of pumice between the application of shellac in increasing 'cuts', i.e. shellac to alcohol ratios. That's how Roentgen or other known 18th century furniture builders would have achieved their glass-like surfaces.

 

Today, one can buy suspensions of pumice or talkum in solutions of shellac (or some other type of varnish) as 'sanding sealers', which is good enough for many applications. And you don't need to take the process of 'French polishing' to the end, but stop at a point that is good enough for the purpose of the model.

 

The fact that shellac will always be redissolved in alcohol can be used to advantage in various applications: you can use it as reversible cement for instance. Shellac simply dries by the evaporation of the solvent, while other materials, such as line-seed or tung-oil, polyurethane varnishes etc. undergo a polymerisation that makes it difficult to impossible to redissolve.