Q-talk 56 - A FINISHING TECHNIQUE
- Category: Q-Talk Articles
- Published: Wednesday, 28 February 1996 06:11
- Written by Ken Miller
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The following article was provided by Robert Bounds. It was originally published in the Central States Association (CSA) newsletter, which supports the Long Eze group. Robert contacted the editor and received permission for Q-TALK to reprint it. The author of the article is Ken Miller. Robert has also submitted an article on his experiences with this procedure which will appear in the next issue.
Ken Miller - Disclaimer: This is only a suggested finishing method. Use your discretion. This article will outline a procedure for finishing your plane with a minimum of work and with outstanding results. Have I got your attention? This process is NOT cheap, but I have found that, when the proper materials are used to do a job, it is seldom inexpensive. I have used this method on two airplane myself, and there are a few more airframes currently in process.
STEP 1: Assuming you have bare glass to fill, scuff sand per plans. Don't sand an area larger than you want to fill in one session. This keeps any unfilled areas from being contaminated in the interim between sessions. We all know that elapsed time between sessions can become quite lengthy -- sometimes years!
Vacuum sanding dust from the area, and then wipe lightly with a paper towel soaked in acetone trying not to spill any liquid acetone on the glass surface. If you pour this solvent on the glass and foam sandwich, the pinholes in the laminate can allow the acetone to dissolve the foam underneath, so be careful. If you are not comfortable doing this, skip this part and go to step two.
STEP 2: Mix a small amount of epoxy (using your preferred system for dry micro fill). Brush on a thin coat, covering entire area to be filled, squeegee off any excess, then use paper towels to wipe off as much epoxy as possible. This will leave the surface dark, but not shiny with epoxy. Let cure until tacky. (I used West System fast hardener for this step, then West slow for the dry micro). This will keep the dry micro from rolling up behind the squeegee.
STEP 3: Mix as much micro as you feel comfortable with. I recommend adding balloons until the surface of the micro ball is almost dull. The dryer the micro, the better, however, the harder it is to spread. Name your poison.
CSA Editor note: If the micro rolls up behind the squeegee cover it with Visqueen and just squeegee over the Visqueen. Leave the Visqueen in place until the micro cures.
Dump the micro onto the area to be filled, then take a 3 or 4" squeegee and spread it out. Don't worry about being too artistic this pass. We just want to get the material spread out so it doesn't try to exotherm as quickly. Now, using your 8" squeegee make as many passes as it takes to smooth the micro, covering the area 1/4" to 1/2" deep in filler. Continue to mix and spread dry micro, attempting to keep the ratios of resin to micro the same between mixes, until the entire area is completely covered. If the stuff begins to stiffen on you before it is smooth, heat the micro just ahead of the squeegee and slowly spread the heated micro. Now is the time to be artistic and smooth as much as possible. I would, as example, fill the entire bottom of the wing in one session. By the time I was through, my fingers were so tired; I needed a day to recover. After these areas are filled, they should look like a huge white-iced cake in the shape of an airplane part. Just assume the whole part is a huge low spot. You can make a whole career of locating individual subtle dips on the surfaces, marking them; locally filling only to discover you missed other low spots, etc. etc. The people who have done this before know of what I speak. If you so choose, when this first and last layer is partially cured, you can mix a small amount of micro and use it to fill any places that pulled up or you missed. Try not to walk away from this until you are sure every square inch is covered. This assures you will have to do this step only once.
STEP 4: Sanding: (BOOOO!) I HATE sanding! Keeping this is mind; here is my tried and true method of moving as much material as possible with each sweaty, grunting, bloody stroke. Go to your friendly auto paint distributor and purchase a good hand-sanding tool. The one that I'm speaking of is about 3" wide and 18" long with a wooden handle the length of the tool and clips to hold the sandpaper. Instead of buying the expensive pre-cut sandpaper for these tools, I buy 40 grit belt sander belts that are at least 24" long and cut to fit. Some places have a bin full of odd length belts that have been discontinued for one reason or another. They can be a bargain. This will save some bucks in the long run because the cloth-backed belts will last 4 to 5 times longer than the paper counterparts. You may choose an air file, which is fine, but I found I could press down harder and remove more material with the hand sander.
Sand until the surface is contoured and you can begin to see the shadows of the glass beginning to peek through all over. At this point you should change to a 60 or 80-grit sanding belt. This is where beginners get into trouble ... not knowing when to stop and call it good. Being verrrrry careful, continue sanding at a 45 until the high spots are just covered by a few mills of micro. STOP!!! If there are still low spots around the high spots don't sweat it as they'll be taken care of later.
Be careful not to sand too deeply in any of these steps. If you come back and re-fill, you will find it difficult to sand only the low area without screwing up the whole thing. This results in having to fill a much larger area to make up for it. Take my word for it, if you are forced to re-fill on top of micro you are almost ahead by sanding the micro down vertically to bare glass in the local area, forming a crater with vertical walls, then filling with micro. If you choose not to do this and you want to fill directly on top of cured micro, do not coat the area with pure epoxy as you would on bare glass. This, when cured and sanded, leaves a ridge around the perimeter of the newly filled area.
STEP 5: After all contours are true and smooth, you are ready for microprimer. This is a combination of epoxy primer and microballoons. This will fill any deep sanding scratches as well as any exposed glass weave that remains. I use the following product: PPG brand K200 primer-filler. This can be purchased at any paint distributor that handles PPG products. I also recommend buying a $40-$50 Binks knock-off paint gun to be used for the primer only. Primer trashes a gun so don't use your $400 Sharpe hypersonic for this job.
The primer is a 4-1-1 mix ... primer, hardener, and reducer. For those first two coats, mix your quart cup only 3/4 full. This takes practice but we want to leave enough room for adding 20% by volume of microballoons. This equates to filling the rest of the cup with loose microballoons. Power mix thoroughly. I use my Makita with a mixing paddle. Put on your fresh respirator with an organic paint filter, open the doors, and spray a small area. Run your gun at high-pressure 60-90 psi with the needle wide open. It it's too dry add a little reducer until it comes out freely. Coat the surface until it is all one color. Not too thick this pass ... just a uniform coat. Clean your gun immediately with lacquer thinner or acetone. If you let this stuff set up in your gun the cost of the process goes up dramatically as you purchase new guns. Allow curing. This time varies with temperature and humidity. Using 80 grit in your sanding handle, sand the primer with the same 45-degree method that you would with the 9 x 9 spline. Some of you may want to use Rutan spline for this step. That's OK and probably a good idea. After sanding, high and low spots will stick out like a sore thumb! Don't let this scare you. The next pass will take care of those areas. If you happen to see a low area that needs micro, after you have sanded the primed area, simply sand down into the low spot, mix some ultra-dry micro and fill. If the area just filled is not too large you don't have to wait for the micro to cure before applying the next coat of microprimer. It will sand down with the next coat. By this time, the surface scratches in the micro should be filled and smooth. If they are not, use some microprimer to spot spray those rough areas. You will have a mottled surface with patches of micro showing at the high spots and sanded microprimer everywhere else. Once you are satisfied that the surface is free of deep sanding scratches, and has an all over smoothness, it is time for step six.
STEP 6: Mix a full spray gun with 4-1 primer and hardener. You won't put reducer in this coat. Spray the surface with a heavy coat, concentrating on any textured areas left from the previous coat. While this coat is curing, pour whatever primer is left in the gun into an epoxy cup. Mix a little micro with it to make a paste, squeegee a small amount of the mixture into any pinholes or remaining scratches. Allow curing thoroughly, then sanding with your sanding tool of choice with 100 grit. When you have sanded the entire area, you should see very little micro peeking through. You are close now! As Vance Atkinson would say, "The fun meter is pegged now!" If there is an area that shows a large area of micro or a concentration of small low spots, recoat locally and sand before going on to final primer coat. You should, by now, have the hang of it. I suggest starting on the bottom of the fuselage first. That will give you plenty of "nobody-will-ever-see-it" practice.
STEP 7: Here it is folks. This is the point when the airframe looks all one color for a change and you can see the end of the tunnel. Mix your primer 4-1-1 for a light fill. Coat the surface with a good heavy coat trying not to sag or run the primer. After cure, this coat will be wet sanded. I use an 8" rubber sanding block purchased at the same store I got the paint. Using 320 grit wet or dry sand the whole surface. If you happen to sand through the last coat into the microprimer, come back after the surface is dry and re-coat. You need only to re-coat, locally, the exposed areas. Next go to 400 grit and wet sand again. This surface is ready for your sealer and topcoat. If you are going to fly the airplane before paint, I recommend a "sacrificial" coat of K200 mixed 4-1-1. This coat is then hand sanded with 600 grit until smooth enough to fly.
Before sealing and topcoat, be sure and clean any oil, grease, bug splats, etc. off with a wax and silicone remover (PPG makes a good one) before sanding. If you don't do this you could contaminate your sand paper with oil and put a nice thin paint release coat all over the airplane. That means when you put your $400 to $600 worth of paint on the airplane, you can peel it off and start over. Get it?
This primer should be comparable with any topcoat you choose, however, check with the supplier of your topcoat for compatibility. It's not a good idea to mix manufacturers' products i.e. if your primer is PPG, it's a good idea to use their product for the sealer and topcoat.
I know this should go without saying but please protect yourself from all these harmful chemicals. Acetone de-fats your skin and causes dermatitis. The primer chemicals are going to be airborne as a fine mist, so use in a ventilated area and wear an activated charcoal organic paint mask. Those paper dust masks are no good for this. It would be a shame to get to the end of the multi-year dream and then keel over dead from over exposure to the chemicals.
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