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Building Tips and technical articles. => Paint and finishing => Topic started by: Brett Buck on July 29, 2014, 11:01:57 PM

Title: Blending in Automotive Clear
Post by: Brett Buck on July 29, 2014, 11:01:57 PM: I recently failed to complete my new airplane for the NATs, so I ended up needing to repair some dings acquired over the years on the old one, along with some intentional sand-throughs of the clear (to get rid of pink spots from raw fuel shortly before appearance judging at the 2011 NATs). The finish is Klass-Kote and K&B Superpoxy colors, and two-part polyurethane clear, as recommended by my 20-point buddies PTG and Uncle Jimby. Fixing the dings in the color was no real problem. The real issue was how to get rid of the overspray halos from airbrushing the clear.

   There are two basic approaches - mask it off and spray to the line creating an edge, that is then sanded to feather out the edge. This works well when there is a good clear delineating edge, or color change. I did this on my turtledeck, clearing to the blue lines, then sanded it to feather it out, and polished it.

   The harder part was where I had to just spot-clear it. I used an airbrush, masked off the area, but then feathered it with short of the mask. The result was a nice, shiny hard surface from the 8-year-old clear, a shiny dead-soft surface in the middle of the repair, and a halo of soft droplets. If you wait for the repair to dry, you end up trying to sand the pebbles and maybe go through the existing clear, or just end up leaving it with a visible halo. I had this happen on several previous repairs.

   This time, I found a better solution. Waiting a grand total of about 2 hours after spraying and just as it was hard enough to keep from leaving fingerprints, I took 2000 sandpaper and medium rubbing compound and started working the edges. It was almost miraculous, it took the very soft halo droplets off like I was wiping a smudge off a car window, while leaving the underlying rock-hard clear with minimal scratches. The remaining solid repair are got pretty matte, so after I got the halo off, I stopped and waited until the next afternoon, then polished it normally, removing the slight scratches in the old area, shining up the matte area of the new repair, and cleaning up the rest of the overspray.

   This all worked out great and the result was that even I, knowing exactly where to look, couldn't tell where the edges of the repaired clear were. They were in the worst possible place, right on the top of the wing in the dark red, where any mistake would stick out like a sore thumb. There was only one small spot where I went through the existing clear.

   I was very impressed with the ease at which this all worked, so it might be worth a try if you have a similar situation. I would point out that this was with the airplane completely disassembled, cleaned, sanded and painted on a Monday about midnight, and I packed it in the car for the NATs Wednesday afternoon, and I wound up with 17 points, which kept me in the game for the rest of the week.

   Brett
Title: Re: Blending in Automotive Clear
Post by: Derek Barry on July 30, 2014, 02:22:10 PM: Good tips Brett.

I need to change clear, I have always used acrylic lacquer. Fuel is constantly ruining the finishes on my planes, especially around the nose. Also the lacquer continues to shrink for a few months. I spent about 4 hours at home and another 4 hours in the hotel room in Muncie rebuffing the Cutlass this year and as soon as I walked into the 180 building I could see all my sanding scratches. I guess I rushed the rubbing and polishing compound stages and in the hotel room I could not see how bad it was. I did the best I could to shine it up minutes before I handed it over to be judged.

Derek
Title: Re: Blending in Automotive Clear
Post by: Randy Powell on July 30, 2014, 03:34:22 PM: I usually shoot the clear over a repair area the empty the gun, fill it with reducer so that there is still about 3% of the clear in the gun and shoot that over the whole area. Tends to sort of melt the new clear into the old. Once it's gone off (about 24 hours for me), I sand the area with 1500 then rub it out and the repair is generally undetectable.
Title: Re: Blending in Automotive Clear
Post by: Doug Moon on July 30, 2014, 04:04:41 PM: Brett,

Sparky was telling me somthing very similar to what you are saying when buffing auto clear. He told me to wait until I cant put finger prints in it and then sand it out right away and buff as soon as possible after that. That way you would buff it before it gets to hard and you have to spend day after day after day just to get it flat before buffing.

Sounds like you kind of worked yours over at the same stage he was telling me to buff out my next plane.

Very good tips. Thanks.
Title: Re: Blending in Automotive Clear
Post by: Derek Barry on July 30, 2014, 04:09:16 PM: Quote from: Doug Moon on July 30, 2014, 04:04:41 PM
Brett,

Sparky was telling me somthing very similar to what you are saying when buffing auto clear. He told me to wait until I cant put finger prints in it and then sand it out right away and buff as soon as possible after that. That way you would buff it before it gets to hard and you have to spend day after day after day just to get it flat before buffing.

Sounds like you kind of worked yours over at the same stage he was telling me to buff out my next plane.

Very good tips. Thanks.

See, that is why I have not tried the two part clears. I need to be able to buff my plane in stages over a couple weeks, I just cant get it all done in a few days. If you wait too long with epoxy clear you cant buff it, right?

Derek
Title: Re: Blending in Automotive Clear
Post by: Brett Buck on July 30, 2014, 04:29:18 PM: Quote from: Derek Barry on July 30, 2014, 04:09:16 PM
See, that is why I have not tried the two part clears. I need to be able to buff my plane in stages over a couple weeks, I just cant get it all done in a few days. If you wait too long with epoxy clear you cant buff it, right?

I don't know, but I have never used epoxy. This was two-part urethane. This particular type can still be rubbed, with a lot of elbow grease, 8 years later. I ended up redoing most of the airplane just last week.

Other types (DAU 75/DXR80) were about the consistency of diamonds after a few weeks. The beauty is, once you get it right the first time, not much is going to harm it, but you sure aren't going to have a fun time if you try it a year later. The type Jim/Phil use is much more forgiving, but also more prone to scratches, etc.

I don't think it is a good idea to do a regular rub-out on the whole airplane without waiting a few days. It definitely does sink a bit over the first day or so, significantly, and you definitely will not be able to get maximum shine out of it if you do it when it is still soft. For the case of removing these halos, you need to get the individual pebbles off, and the difference between the very hard underlying clear and the very soft repair makes it a snap.

Of course, I am just a hack (as someone pointed out during the NATs), so maybe I accept less-than-deal results.

Brett
Title: Re: Blending in Automotive Clear
Post by: RC Storick on July 30, 2014, 05:55:55 PM: Brett I have to disagree with you on the paint shrinking. I know whats new. Catalyzed paint gets harder the longer you wait. Its not like dope at all and will shine just fine after waiting a few hours and then sanding it. The longer you wait to machine buff it the better it shines. But I start sanding as soon as I can touch it. Its worked for me on airplanes and motorcycle tanks for years.

In the case of halos I use thinner but they sell a blending agent. All this does is melt one soft coat to the hard one. Then sand and buff. I did a demo movie on my technique on YouTube
http://youtu.be/p4PmJK6sTco

I used ope as the clear but this same method is used for urethane
Title: Re: Blending in Automotive Clear
Post by: Brett Buck on July 30, 2014, 06:17:59 PM: Quote from: Robert Storick on July 30, 2014, 05:55:55 PM
Brett I have to disagree with you on the paint shrinking. I know whats new. Catalyzed paint gets harder the longer you wait. Its not like dope at all and will shine just fine after waiting a few hours and then sanding it. The longer you wait to machine buff it the better it shines. But I start sanding as soon as I can touch it. Its worked for me on airplanes and motorcycle tanks for years.

In the case of halos I use thinner but they sell a blending agent. All this does is melt one soft coat to the hard one.

I have used the thinner and blender methods in the past, and they were OK but not nearly as controllable. I was very happy with the results on these latest repairs. In fact I used the thinner method on the color in a few spots this time. It actually works pretty well with Klass-Kote thinner, because it is very slow to evaporate.

I haven't had any luck at all using thinner blending with dope because by the time you get to the paint, the thinner is gone. It might work with retarder but I am scared to be spraying that directly. Of course, you also don't really need it.

Brett
Title: Re: Blending in Automotive Clear
Post by: FLOYD CARTER on August 06, 2014, 11:05:08 AM: I have been using a two-part automotive clear coating. The big problem, as I just discovered, butyrate dope cannot be used to repair covering tears or dings over this clear coat. I didn't try sanding the clear coat first around the damaged area, but I suspect it is so hard that sanding would be quite a chore.

Has anyone found a way to do this?

Floyd
Title: Re: Blending in Automotive Clear
Post by: RC Storick on August 06, 2014, 11:45:22 AM: watch my video above