‘Universal’ Primer

[CTYankee]

Good day all-

 

In my other thread:

 

@druxey suggested i use ‘universal’ spray primer.  Can someone clarify what that is? I see a bunch of references to Tamiya, but i thought tamiya was acrylic and I am not sure if that will be compatible with the enamel paints i plan to put over top.

 

In a different thread @wefalck mentioned the paint systems need o be compatible, so I am bit nervous. I have already spent hours un-borking my previous boo-boo.

 

(I started this new thread to help newbs like me find the answer to this question in the future. I’m sure I’m not the only one who is- or has been- mystified)

 

 

[wefalck]

Paints are funny things and a lot can go wrong, when one begins to mix different things. A stirred paint is a (usually) carefully designed suspension or dilute gel of pigments in a mixture of 'medium' (e.g. lineseed oil) and solvent (e.g. turpentine). If you change the ratio between medium and solvent, the suspension can break down, it flocculates. Many commercial paints are quite forgiving as to the ratio as long as you use the right solvent or medium. If you are adding a fourth component to the system, i.e. a different solvent, a lot of things can happen: the paint may coagulate, the the medium breaks down with the consequence that the paint does not 'dry' or bind (the pigment dust off), etc. This is why insisted on the compatibility in the other posts. Such things can also happen, when certain paints come into contact with certain incompatible surfaces, e.g. some metals, such as copper, or different primers or other types of paint. The new paint will physico-chemically react with the surface, which can lead to its breakdown.

[Jaager]

Shellac is as close to being a 'universal' primer as can be had.    It is available as flakes and already in solution.

The flakes are for when a clear finish is the goal and no darkening of the wood is desired.  The solvent is alcohol.  Super Blonde is close to being water clear,

 but the solubility in alcohol is about 1/2 that of Garnet.  If a paint is going over it, it is easier to just purchase is premixed  and dilute it.

The first coat should a half strength concentration .

The hows and whys of using shellac have been discussed more than once in this forum.  Bob Cleek has done the best job of explaining, 

It is a very old and times tested material so it has old measures   - pounds cut - which is pounds of flakes per gallon of alcohol.  I think garnet is 10 lbs cut for saturation which rounds out to being a 10% solution.   Battery operated small scales are not expensive,  so it is easy to weigh out 10 gm and qs to 100 ml with shellac thinner or  5 gm of super blonde and qs it to 100 ml with shellac thinner.

[Bob Cleek]

6 hours ago, Jaager said:

It is available as flakes and already in solution.

The flakes are for when a clear finish is the goal and no darkening of the wood is desired. 

 

The color range of natural shellac runs from orange which, with multiple coasts, will appear dark brown. Shellac flakes are bleached to achieve varying darknesses ranging from "clear" on up. You can obtain flakes from mail order houses and dissolve them in your own alcohol, but buying premixed "two pound cut" shellac in quart cans is cheap and easy. (Most paint and hardware stores carry it. In the US the brand is Zinsser "Bulls Eye.") It comes in two types, "orange" or "amber" and "clear." For modeling purposes, the clear is what you want. If you wish to thicken it, simply take some out and let the alcohol evaporate. To thin, just add more alcohol. A quart should last you a long time. Brushes clean up with alcohol. I store my used brush cleaning alcohol in closed containers and start washing a brush by rinsing in the "dirtiest" jar of alcohol, then move to another that's a bit less dirty, and finally rinse in a jar of clean alcohol. As the alcohol in the jars gets progressively dirtier, it gets moved into the "next dirtiest" jar. I use the "dirtiest" used brush cleaning alcohol for adding back into the can of shellac to thin it as needed, or even, if there's enough shellac dissolved in it, I use it for when I want very thin shellac. You save a lot on alcohol that way and don't waste shellac, either. One can also always leave the "dirty" jar of brush cleaning alcohol open for a day or two and let the alcohol evaporate and yield shellac of a desired thickness. Shellac that is "molasses" consistency is sometimes used to good effect as an adhesive. Dried alcohol is easily removed by simply washing with alcohol. Shellac permeates the wood surface and does not produce brush strokes or raise the grain. It dries flat on bare wood, but with multiple coats, will build to a gloss finish. It sands easily to a very smooth surface. Shellac is an archival material. Three thousand year old shellac covered artifacts in good shape have been found in Egyptian tombs. And it's really cheap compared to other coatings on the market. It's non-toxic (aside from the denatured alcohol it's mixed in. Shellac is what is used to coat jelly beans to make them shiny. What's not to like?

 

Shellac contains a natural waxy substance that supposedly interferes with the adhesion of polyurethane and water-based (acrylic) coatings, although I've never encountered any problem applying oil-based finishes over dry shellac. If one is concerned about this, Zinsser also sells "SealCoat," a "dewaxed" shellac for use beneath polyurethane and water-based finishes. "SealCoat" is advertised as a "universal sanding sealer" for this reason.

 

(Don't waste your money buying shellac in a rattle can. If you don't want to pay sixteen to eighteen bucks for a quart, you can buy a half-pint for ten bucks.)

 

See: https://www.rustoleum.com/product-catalog/consumer-brands/zinsser/interior-wood-finishes/bulls-eye-shellac/

 

[wefalck]

I love shellac and have used it a lot over the past 40 years or so on furniture and modelling projects. However, I would not necessarily call it an 'universal' primer.

 

Its applicabilaty depends very much on what you are applying it to -> compatibility.

 

Bare wood is ok, but as soon as you may have applied some putty or filler, the situation changes, when these are soluble in alcohol. Dito. any other surfaces that are attacked by ethanol.

 

Sometimes, it is desired that the primer attacks the surface, because then it is able to key in stronger, one of the main functions of a primer. However, particularly when applied by brush, you may bring so much solvent onto the surface that you are softening and distorting it.

 

Tests are always advisable.

[Bob Cleek]

1 hour ago, wefalck said:

I love shellac and have used it a lot over the past 40 years or so on furniture and modelling projects. However, I would not necessarily call it an 'universal' primer.

 

Its applicabilaty depends very much on what you are applying it to -> compatibility.

 

 

Zinsser's "Universal Sanding Sealer" (de-waxed shellac) is a different product from plain shellac. I've never used it, but according to their product description, the removal of the natural wax from the shellac in this product permits this coating to adhere well to previously coated surfaces. Like any product with alcohol used as a solvent, it's applied to alcohol-soluble coatings, it will soften the coating beneath it, of course. I only use shellac as a sealer on bare wood. I sand smooth and apply a single liberal coat of shellac, which soaks into the wood. When the shellac is dry, I give the surface a final sanding with fine grit sandpaper, and then apply paint over the surface, or if the wood is to be left bright, I just leave it without any further coating. 

 

See: https://www.rustoleum.com/product-catalog/consumer-brands/zinsser/interior-wood-finishes/sealcoat-universal-sanding-sealer/

[CTYankee]

Thanks all for the valuable feedback here! Lots to think about.