April 30th, 2012, 03:46 AM
I'm busy doing my first guitar refinishing and I just ran into a funny issue :
I bought a varnish spray can to use for the clear coats and it's based on alkyd. Tried it on a sample and how funny, the coat leaves a noticeable yellowish cloud after drying (or curing whatever, I don't know the difference).
After some research I see that Alkyd is oil based, which is probably the reason for this stain.
While I found the result disgusting at first, I'm now thinking that it would make for a nice fake aging effect. I might use it on the guitar after all ...
I see people also use water based varnish (or lacquer, I don't know the difference either lol) but I can't find any in my local home depot.
Looks like I don't have options really ...
Anyone got experience with Alkyd based finishes ?
At least I hope it gets hard enough after drying and I won't run into problems when sanding and buffeting.
April 30th, 2012, 11:24 AM
Alkyd's are poly type finishes, and, as far as I know, are exclusively oil based products.
It sounds like you might be experiencing some blushing - that is water vapor being trapped in the finish. Aerosols are more prone to this because the pressure release associated with the spraying causes a temperature drop and then the temperature drop can cause humidity from the air to condense into the finish as it is applied. It usually (but not always) clears with time.
A simple approach that can help minimize this is to gently warm the contents of the can (in a water bath) just prior to spraying. The slightly warmer mist being less prone to condensing out ambient humidity. The downside is that the warmth also makes the spray slightly less viscous so more prone to runs.
On the upside alkyd resins tend to be very durable. On the downside, being rather flexible, they can be difficult (if not impossible) to polish to any degree of gloss.
They also tend to yellow with age (this can be good or bad depending on you needs.)
May 2nd, 2012, 05:06 AM
Thank you for the information. Looks not like a winner then ...
I might try some parquet sealer instead, those are PU based I think.
I know it has been used for guitar finishes by some people already and it seems also suited for thick coats.
May 2nd, 2012, 06:09 AM
You are on the right track , try to find a trade supplier of commercial flooring varnishes / lacquers. They usually have aerosols of what is called Pre Cat.
That will work well for you.
They also usually have a selection of low VOC water based lacs for hardwood floors that are UV resistant so will not yellow.
Also they usually have what is called Acid Catalyst lacquer and it is similar to the pre cat in the aerosols.
You can achieve a very good finish with this but without spray kit you have to apply it by hand. I have used no more than a wad of old T shirt to do this and it works fine. You build up a few coats and then wet sand it flat and then polish.
It cures very quickly.
May 3rd, 2012, 04:47 AM
Thank you. I'll definitely have a look on the flooring varnishes available around here.
May 4th, 2012, 02:13 PM
I wouldn't get hung up on the type of resin, even though for marketing purposes it's usually the largest word on the can. Traditional varnishes were made with oil (usually linseed oil but sometimes more expensive tung oil, an important part of the curing process and sometimes used alone as a finish), a solvent (such as naptha, which makes the mixture fluid enough to apply it with a brush or rag or sprayer), resins (which become solids when the mixture cures, examples are phenolic, alkyd, and polyurethane, but there are also acrylics and polycarbonates), and other additives (such as driers - remember lead in paint? -- or UV absorbers). With all that going on it's not that helpful for the label to scream "polyurethane" when that's just one ingredient.
Having said that, there are properties that each type of finish will give you. An old fashioned varnish with phenolic or alkyd resins in it will dry to an amber color, whereas polyurethane won't have quite so much color in it. Another property to be concerned with is that "varnish," while easy to apply (it dries so slowly that brush marks and bubbles will find there way out before it hardens), is not as hard as other finishes that you might find suitable for a guitar.
May 7th, 2012, 03:27 AM
Thanks for the precious info.
I admit I'm totally lost in the descriptions on the stuff.
I've bought a tin of floor varnish that says "water based acrylic PU" that would be used for wooden stairs. Going to apply it with a brush.
I had to wet sand the last coat a bit with 600grit since I had one small run, though I didn't remove all of the orange peel. I heard the clear coat is supposed to remove that effect.
And I admit it's a bit frightening to apply a clear coat over a sanded/dull finish ... I suppose the varnish is going to "wet" it and make it shiny again.
Well I sure hope lol
One last thing is I wonder how long I should wait between clear coats.
The tin says dust free after 4 hours, dry after 24 hours ...
If I applied say 14 coats that would take 2 weeks ? Really ???
Also some people say you don't need to sand between coats, other say you have to ?? Since I'm a lazy guy I was hoping to just sand the last coat up to 1200/2400 grit and then polish it.