Beginner question: Achieving a nice finish

[Gary Nutt]

I have been using water-borne finish (KTM-9) because of the difficulty in using lacquer in a safe manner. I do not have experience (say, as a cabinet maker) in producing really nice finish. My finishes come out kind of blotchy -- some parts glossy and other parts a bit dull. I wonder if I am not getting the surface flat before buffing? This seems kind of difficult with wood that is a little concave/convex due to bending, clamping, etc. Any hints/wisdom/common sense you can share with me?

Also, it seems like when I tried wet sanding on the KTM-9 after it had set up for 2 weeks, the finish quickly dulled -- is that what it is supposed to do? Or do you have to wait more than 2 weeks to wet sand?

Thanks in advance,
Gary

[Greg Mirken]

Caveat: I have no experience with KTM-9, but a fair amount of finish experience. I don't think a non-flat surface has anything to do with your blotchy outcome. Are you using compound and buff recommended for this finish? Material is fresh, mixed thoroughly and applied correctly?

Wet sanding should produce a very even, level surface with a tiny scratch pattern that feels very smooth. Is your paper loading up? If so, some areas of the finish aren't curing properly.

I'm sure someone with water-borne experience will chime in. If you can get a meaningful picture of the problem, that would doubtless help.

[Gary Nutt]

Greg,
Thanks, I will look elsewhere (other than worrying about non flat surfaces). I think I am using the compound and buff recommended for the finish -- I am using Menzerna compound with a buffing wheel. Of course I am not so sure about the "applied correctly" :-) I haven't been wet sanding because the few times I tried to do so, it seemed to dull the finish considerably. The paper did not seem to be loading up, though.

Here are two pictures of backs from recent rosewood guitars. One of the interesting things about my problem is that the lack of uniform gloss is pretty hard to photography, but is quite apparent when you look at the guitar in good light (this causes some disappointment to buyers). That is, the lack of uniform gloss is usually most evident by the amount of light reflection. In the first picture, some of the problem is evident on both sides of the backstrip (although the problem is pretty much all over the back). In the second picture, the duller areas show a little in the waist area.

[Blyth]

Gotta wet sand! It will look very dull when you're doing it, but after a few more coats and buffing, it will most likely remove those splotchy areas. I can almost guarantee that this is your problem.

[Gary Nutt]

Thanks, I'll try wet sanding. I had tried it earlier, and it seemed to dull the surface even more (the surface had cured for a couple of weeks, but maybe that is not enough?)

[Bob Gaddis]

WIth any finish prep work is more important than than the actual finish. You don't say what type of wood you're finishing, if you use filler or sealer, are you sanding or scraping, etc., so it's a little tough to speculate what problems you're having. How are you applying it? Under what conditions do you apply it? Is the finish properly reduced if you are spraying?

[Gary Nutt]

Bob,
This is rosewood, and I have had the problem with various types. I have tried both epoxy and LMI micro bead paste filler. Then I seal with a light coat of the KTM9. I sand lightly between coats if there are bumps, etc. I have not been able to spray the KTM9, so I have been brushing it. It is applied in a closed room at temperatures between 70-80 degrees, 35-40% humidity. After the finish cures for a couple of weeks, I use Abralon on a random orbital sander, starting with 500 and working up to 4000 grit. Then I buff with Menzerna -- I used very fine and gloss, but more recently I have used medium and fine. Sometimes I think I am just not buffing enough, or maybe I am being too stingy with the compound?

[Bob Gaddis]

Since rosewood is a resinous or oily wood, are you using a solvent to wash the wood before filling? then are you burnishing the filler by vigorous hand-rubbing? Are you letting the filler thoroughly dry and then sanding, followed by a tack-rag and a naptha cloth to remove all vestiges of dust? My only experience is with nitrocellulose lacquer, so I can't speak directly about the KTM9 products. Do you brush it thinned or what?

Are you using a wheel-buffer or a hand-buffer?

[Gary Nutt]

I have not used a solvent before filling (other than naptha), although I have burnished lightly -- I wouldn't say I had vigorously hand-rubbed after applying the grain filler. I have let the filler dry thoroughly, then used naptha to remove dust before applying finish. I thin the KTM9 before applying. Finally, I am using a wheel-buffer.

It sounds like I need to exercise more care when filling the grain. I am surprised (but I certainly believe you) that these errors at grain fill time would result in appearance I am seeing, i.e., some areas buff to a high gloss and others areas remain sort of dull.

Do you think it would help to use steel wool -- I guess before using Abralon, or maybe before buffing? Whatever error(s) I am making, they are being repeated each time I do the finish. Thanks for the advice.

[Martin Edwards]

Gary, Finishing is the bane of most hobby builder's lives.

there are a lot of folks who outsource finishing because it's such a pain.

Personally I take the easy way out and steel wool to a semi matte finish.......

[Ronnie Nichols]

I haven't used any of the waterborne guitar finish. I am an old nitro lacquer sprayer who was raised around an auto body shop and sprayed a lot of lacquer on automobiles. This background has been a real help in instrument finishing for me. The photos look like the finish is gone in the areas I see. Are you sure you applied enough finish in the first place? It takes a pretty good film of finish to fill in the pores of rosewood. I've tried all kinds of products to fill the grain and it still seems like you have to really load it up to accomplish the gloss without sanding through. I realize some of the newer finishes have a higher build and don't require as many coats. With lacquer, I never start sanding until I have applied 8 or 10 coats. You can recoat lacquer because one coat will stick to the next. You don't want to put too much finish on but you might consider applying several coats before you sand. A good lacquer finisher minimizes a lot of the final buffing by carefully sanding between applications (not necessarily between each coat) with the proper sandpaper. I've intended to try some of the waterborne finishes for a while but I just haven't done it. Lacquer is pretty forgiving and I know how to deal with the problems that arise. The overall proceedure should be basicly the same with the waterborne finishes.
Ronnie Nichols

[Gary Nutt]

Thanks for the advice. I have wondered if I was sanding through -- I don't think I am going through, but I sure the heck might be getting it too thin. The first couple of times I used KTM-9, I applied too much, so the finish looked pretty bad. I have lightened up the amount of KTM-9, both by thinning more, and by putting on fewer coats. BTW, you can recoat KTM-9 too, because it cross links. I don't think you can wait an arbitrarily long time to do it., but certainly you don't have to do any prep to coat within a day or so. I have been applying about 6 coats of KTM-9, because it seems to be much thicker than lacquer. I did try lacquer a few times on my first couple of guitars, but I really had no good place to apply it. I realized that I was flirting with danger -- both because of the fumes and its volatility. I used it a few months ago on a repair. I'll try sanding a bit more between stages.

[Gary Nutt]

A follow up: Months after writing this, I finally discovered that Ronnie Nichols had correctly diagnosed my problem -- I was not putting enough finish on the guitar. 6 light coats of KTM-9 is not enough finish.

Thanks for the help,
Gary