Building Tips and technical articles. > Paint and finishing

A Few Thoughts on Finishing

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Randy Ryan:
One of the things we often hear about is the weight the final finish adds. Now I haven't used many products other then dope and some rattle can paints for my models, so I can speak with some authority on Lacquer based finishes. Regardless, the preparation for the finish is probably of the greatest importance.

This post was sort of triggered by Ty's mention that Brasso is a rubbing compound. That is basically true, but the quality it has over and above compounds made for paint polishing I think is in its grit size and lubricity. That said, what follows are the suggestions I make and I have proven them on myown models.

1. No matter what you cover with, the finish begins before it gets anywhere close to the model. Attention to fits and level joints is paramount. Sure, there are good fillers out there, but here lies the first batch of calories. Balsa is lighter that any filler I've ever used, and I have yet to find a filler that sands the "same" as balsa. Because of that, it will always read through the covering and the only remedy is what else, more filler. So, here is where you can start you weight watchers program, right down to paying close attention to wood thickenss because 1/16 balsa for example will vary from .01 under to .01 over. If for instance you do your sheeting with some that's thick, and cap strips with some that's thin, it will require that you sand a bunch to level the joints. This in turn will cause bumps in the sheeting over all the ribs because the sheeting is not supported between them. Careful selection of not just quality, but thickness will save allot of time and uglyness.

2. OK, you've built the perfectly fitted and joined airframe and sanded it well, now what. Give it a "bath", wipe it down with a wet cloth and leave it to dry. This will raise out small dents and areas that were compressed rather then sanded smooth. In 4-6 lb stock, its very easy to simply compress and burnish the wood, but if that's what you've done, it'll come back and bite you down the line. When its all dry, carefully and LIGHTLY go over it with nothing rougher then 400 grit. Cut the sand paper into ovals about 1 1/2 wide by 2 1/2 long, curl it over the edge of the bench grit side up. This will make the edges tend to pull away from the surface rather then dig in. Still you'll have to be careful. Remember this from now on, you'll want to use it the rest of the way through. If you're using iron on, you're ready to cover, if silkspan, silk or tissue, you're ready to dope. Now the tricks get more subtle. I use all of these materials, but I will concentrate on span although these tips will work on any doped on covering except synthetics. The main areas to watch are the wing, stab, and rudder tips and other compound curved area where you use slits to make the covering comform. When you do that, you double the thickness in areas and that's not necessary. Get a couple coats of dope on the whole thing then start sanding with your little ovals. From this point on, nothing rougher then 400 should be used. concentrate on the lapped joints and even use a couple extra coats to seal them sanding in between to feather them out eliminating the need to apply filler later on. This can be done on the LE, TE and fuselage seams too, but be careful not to sand through. From about 3 coats on, sand lightly knocking off nibs and raised fibers, but be WAY careful if you're using silk. I normally use about 6 coats of full strength dope before I prime. What you want to do is make the surface as perfect as you can BEFORE you prime.

4. Priming. Here is where some will part company with me, but that's OK, I've proven my point to those that have seen my models, so I won't argue. I am a big fan of Duplicolor rattle can primers. They make a filling primer and a basic sandable primer. No matter what you use, the principal will remain the same. The prime should not be seen as the opportunity to fill small imperfections, you should have already taken care of that. From the first coat of primer never use anything rougher then 400. You need to some roughness here refered to as "tooth" to give the color something to latch onto. I can't stress enough how important this phase of the finish preparation is. Sanding can be tedious, but it is necessary, you do not want to be filling imperfections with clear at the end. Remember the paper ovals. Sand fillets with rolled up paper, and oh yes, this is one place you may want to go just a bit rougher, I use 320 grit on the fillets. DO NOT use progressively fine grits to get to 400!!! Start and finish with it. Progressively sanding with finer grits really just takes longer, and the fact is, you will only be trying to sand away the scratches from larger grits and in fact will miss allot of them. If you have done the building and covering well, you will notice that the primer sanding goes quickly and levels instantly. By holding up to the light and looking down the surface you should see one unbroken continuous surface. If you see irregularities and the primer is intact, sand a bit more. If the primer is gone, SPOT prime, not need to add where its not needed. You're looking for that all over sheen when you look across the surfaces.


5. Color; use only as much as you need and NO MORE! If you use silver, then give it a coat of clear before you go on to trim, this will seal and bind the pigment and keep the silver pristine. Trim edges, here's another place to cut some serious calories. Don't be afraid to lightly feather the masked edges of opaque colors, even translucents can be feathered because they are generally heavier at the masked edge and therefore darker, but be CAREFUL. Use 2000 grit for this, and do it wet. I'm not talking about massive sanding here, mostly just a rub down, You will not get a total feathering at this point, but you WILL knock off the raised edge that takes some much clear to fill later. OK, now you have all your trim done and lightly feathered. Ready for clear coats right??!!! WRONG!!!. Now get your air brush good and squeeky clean and carefully put 2 or 3 thin coats of clear on all the tirm edges, 2000 grit between coats. You will be amazed at how fast the edges disappear and the surface levels. Carefully and LIGHTLY go over the entire model with 2000. Once again holding up to the light and looking down the surface you should now see one unbroken continuous surface. No little shiney spots, stippling, no little hollows.

6. Clear at last. Here is where we tend to get carried away. We use clear coats the try to fill all those ewglies that read through. If you look at my Sabre Stunt thread you'll see the finished product. This model has only 2 coats of thinned clear on it. A tad bit of retarder to let it flow a bit. After the first coat, it was merely "rubbed down" with 2000 grit. After the second coat it was thoroughly sanded with 2000 grit wet and polished with Brasso. This model tips the scales at 38 0z using an OS .40FP, so you can see that the finish weight is down.

So you see, the "trick" of the good finish is really many little ones along the way. We are always anxious to see and fly the finished product, but don't let that make you skimp along the way and cost you allot of work and flab in the end.

Edited for MANY typos, AGAIN.

Randy Powell:
Randy,

I vary from you only slightly. I use finishing compound at the end and hand glaze. I hadn't thought about wetting the airframe before, but then, I live in a high humidity place. <grin>

I've used rattle can lacquer primer before and it works well. Unfortunately, I live in communist Washington and it's not available anymore. So I'm sort of stuck with other alternatives. I've used K-36 and that works pretty well, but you have to be aggressive with the sandpaper. The stuff is heavy, though you can have dope both over and under it without problems. More recently, I've take to just making my own (clear, filler material, a bit of toner to make it gray. Again, works OK, but I wish I could still get lacquer primer.

Bill Little:
Randy R.:  Great theme, and totally true.  As Sparky says, the finish begins with the first piece of wood cut.  I recently did a model using nothing but tinted Nitrate as the filler.  Lightestr finish I have ever put on!  Would have been outsyanding (my best ever) but I got in a hurry and used too much retarder (WAY TOO MUCH!) in the color and pulled off paint all over the place. (See the "Paint Job from Hades" thread).  But i had the framework totally smooth and flat before I started putting on color.  No grain or imperfections anywhere to be found, and only *filler* was the fillets.

Randy P:  Duplicolor has a new line of totally VOC compliant lacquers available now in their "Body Shop" series.  We are in the process of painting Aaron's T-Bird I using this *lacquer*.  So far it has been OUTFREAKINGSTANDING!  It comes in prethinned quarts (still need to add thinner ansd a touch of flex additive) and dries quicker than the old lacquer.  No compatibilty issues with Sig dope or Dupont A/L.  I think I am going to love this stuff.  I can buy it about a 1/2 mile from my house, and it thins with Dupont 3608S.  Flows great.  Going to do a test shot with Sig Lite Coat as a clear finish.  So far there have been no problems, and it is around $15 a quart.  Have well over 1/2 the can left with the plane completely covered in *White*.  This was over a "speckled" Duplicolor primer finish.  They also have a clear and primer in this "System".  Don't have a clue as to how *Fuel Proof* the Clear is but I will find out on an old beater! ;D

Clint Ormosen:
Randy, Im trying to follow your posted method for finishing. You listed Duplicolor as the primer you use, but I want to use Sig dope for color and clearcoat. Are the two products compatible?

Greg L Bahrman:
Randy Ryan,
Thanks for taking the time to type it out and post it for us. I really appreciate it. Your planes always appear top notch. Thanks again.....

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