Michael Dresdner

straight talk about wood finishing

Not so deftly finished

Q: I decided to spray Deft’s new waterborne acrylic wood finish for the top coat on a guitar. I am not happy with the outcome. I seem to get small fisheye no matter what I do. I am spraying at about 35 pounds and thinned it about 25%. There are no contaminants and I have level sanded after each coat but now it is getting to be a real headache as it doesn’t seem to really improve.
A: Bear with me; you’ve raised a lot of potential trouble spots, so let’s go through them step by step. First, I will once again reiterate that one should not try new materials, or new brands, on an actual project. Finish firewood first.
I don’t know if you have sprayed other brands of waterbased materials before or not, so for the nonce, I will assume you have and take it from there. How many pounds you are spraying at tells me nothing unless you also tell me what kind of spray rig you are using, so I can’t really comment on that portion. However, I will point out that unlike most brands of waterbased coatings, which are clearly designed for spraying as well as brushing, Deft goes out of their way to make it clear that their waterborne acrylic is meant to be brushed. This may or may not be germane to your problems, but it is certainly worth noting.
As for thinning it, I don’t know what you thinned it with, but for the most part, waterbased coatings meant for spraying are designed to be sprayed right out of the can, and thinning can create a world of problems. Solvent based coatings are fairly simple systems, and can be thinned indefinitely, but waterbased are not. They are a delicate balance of components designed to keep resin in suspension in an unfriendly environment; water. Changing that balance in any way can throw the mixture out of whack. For instance, the dispersal agents and defoamers are based on the ratio of water to solvent to resin, and changing that ratio by thinning can make the whole system go haywire. Thus, thinning with water is out, and even thinning with primary solvents, like glycol ethers, can have deleterious results.
Ignoring all that for the moment, there are two other issues that jump out at me. One is the comment that there are no contaminants. That’s almost impossible to know, since most are invisible and do not show themselves until after spraying. When present, they show up as fisheye, which is exactly what you said you have. Bear in mind that some contaminants can come from the sanding media you use between coats. For instance, white gray self lubricating sandpaper can leave stearates on the surface after sanding, and that can cause fisheye.
Here are some suggestions from the raw wood up. Consider adjusting your waterbased finishing schedule to include them all and I’ll bet you have a lot fewer problems.
Seal the raw wood with Zinsser SealCoat prior to using waterbased finish. It helps seal in any contamination in the wood itself, and waterbased finishes flow out nicely over it. After that, use only gold type stearated sandpaper between coats, wipe down only with a damp cloth (not tack rags), spray unreduced finish whenever possible through a very small fluid aperture (1 mm is ideal for most waterbased clears), and spray lightly, even if it looks at the time like you are getting a bit of orange peel. If you do all that and still have problems, consider trying another brand.