Bumps in finish. - created 07-02-2006

Michaud, Joel - 07/02/2006.16:49:37

Hello Iam currntly finishing my first acoustic guitar with ktm-9 water based finish using a spray gun. As i let the finish dry i notice that there seems to be thousands of little bumps all over the surface. With some effort i can get these bumps to sand out, but they always reapear after i put on another coat. Iam just wondering if this is normal? Or if perhaps iam doing something wrong? Any guidance would be awesome!

Thanks!

Did you raise the grain and sand back before you applied the finish?

Yes i did do that!

Thanks though!

Dust?

This may sound like a dumb question, but is it orange peel or different than that. If it is excessive orange peel then it may be normal or you may need to change some of your spraying technique to lessen the amount.

1. I was thinking it might be dust as iam just spraying in garage, but i did try to get rid of all the dust before hand.

2. What is orange peel? What causes it?

Thanks for all the help hopefully i can figure this out! But its not normal to have all these shinny bumps then?

Thanks.

Orange peel is a peebly surface texture that you get for several reasons, with the result that the lacquer starts drying out before it can flow and smooth out. It is different than the very fine satin texture that some waterborne lacquers show when dry, but before final sanding and buffing.

Provided the texture sands out between sets of coats and there is no issue with trapped bubbles in the finish, you should be OK. I'm sure someone with more experience spraying waterbornes can weigh in here if you tell us what spray setup you are using, as well as what temp you are spraying at and whether anything has been added to the KTM-9.

Joel, examine your finish closely, with a magnifying glass. My experience with dust is that it's generally visible as specks in the finish before you start sanding.

Orange peel is exactly what it sounds like. The finish has tiny bumps all over it and resembles an orange peel. It's usually a product of poor spraying technique. I don't know enough about spraying to tell you what to do to remedy your problem though.

When I was doing a finish with rattle cans, I had orange peel. Turned out I just needed to move a bit closer when I sprayed.

Orange Peel is a fact of life in lacquer finishes. It's just a matter of getting it under control as best as you can. You can spray it on like glass, but as it dries it will shrink and after it's all dry you'll see that it isn't smooth like that anymore. That's why we have to wetsand it, and polish it out.

I like to use low pressure, and put it on there thin and smoothly, softly. If you blow it on too thick and too fast you'll get a lot of heavy orange peel. Easy does it, with wetsanding between coats.

I think of orange peel as craters instead of bumps. Bubbles in the finish can cause this kind of bumpiness. Look closely with a loup or magnifying glass to really see what is going on.

Craters seem more to be fisheye, while orange peel looks just like the nubbly peel of an orange, but waterbase could be very different than solvent finishes.

Iam am sprayuing inside a garage, with an uncontroled temperture. I know its not the most professional but its all i go to work with right now. Looking at the finish it does seem kind of like a orange peel like texture. I will look at it more closely with a magnifying glass though to see if i can see anything. If it is orange peel is there anything that i can really do to lessen it? Or will it just sand out with a little bit of effort?

Thanks for all the help guys!

If it's hot and dry then you might have the same problem I was having here:

Mike Dotson, "Great balls of Fire! (OK, just a hot weather lacquer question)" #, 10:47pm Jun 24, 2006 EST

If it is orange peel, which I suspect now that it is, then you need to experiment on some scrap for a while and see what combination of distance from the work peice and speed that you move your gun will give you the minimum amount of OP with the most amount of build that you can successfully spray without runs. It takes experience and patience more than anything else. Oh yeah, and lots of sandpaper