Sealer Tests
A couple of weeks back there was a thread about using the new KBS urethane as a sealer and surfacer instead of other more common materials. I was, at the time, concerned that the urethane, being rather thick and sticky, wouldn’t make a decent substitute for dope. So ….. I decided to do some testing of different sealing and filling materials. The results were interesting ….. bear in mind though that these tests were done by someone who historically doesn’t have a good track record of having very light airplanes, and it’s been a while since I had anything in the 18+ point row. So … if you’re concerned, test it yourself to make sure.
The above mentioned thread said, “ to brush on two coats of KBS”. It didn’t say thin, or scrape off, or wipe off, just brush on. I was skeptical, thinking that the KBS would be too thick and heavy. I had to do some experiments. So …. I made up some 1 foot square test boards of 1/8 th inch balsa to test different sealing methods. The surface of the boards wasn’t finely sanded, just raw balsa. The results are listed below.
1. Brushed on two coats of KBS urethane (fairly stiff bristle brush not foam)
5.5 gms/ft**2 first coat on raw wood brushed on with fairly stiff brush, straight out of can
3.6 second coat applied with stiff brush, straight out of can
2.6 after sanding w 100 grit paper
Surface glossy after second coat … appears filled pretty darn good after sanding surface with 100 grit. A little filler and you’re done.
2. Brushed on two coats of full strength dope (Certified butyrate low shrink)
10 gms/ft**2 first coat on raw wood …… holy cow, straight out of the can this stuff is unusable …. Way to thick to brush.
3.3 second coat was thinned to brushing consistency .. not such a good start for the tests
4.3 carbon veil applied with tinned dope
Standard looking surface ready for filling.
3. Brushed on dope , thinned to good brushing consistency with carbon veil
2.7 gms/ft**2 first coat on raw wood
1.8 second coat
2.1 third coat … lite gloss
5.8 0.2 carbon veil applied with dope
3.2 fourth coat of dope brushed
Ready for light filling
4. Scrape on KBS urethane (brushed on then credit carded off completely)
3.5 gms/ft**2 first coat on raw wood (squeegeed off with credit card)
2.5 second coat (squeegeed off with credit card)
Dry surface but ok for filling
5. Scrape on KBS urethane with 0.5 oz glass (brushed on then carded off)
8.1 gms/ft**2 first coat on raw wood with 0.5 glass (squeegeed with credit card)
2.9 second coat (squeegeed with credit card)
Dry surface, about like previous but strengthened with glass
6. Brushed on K&B Superpoxy with satin finish hardner and glass
8.1 gms/ft**2 first coat on raw wood, K&B + satin finish hardner + thinner (1:1:1) plus 0.5 glass cloth brushed down
1.3 second coat, K&B +satin finish hardner + thinner (1:1:1)
Ready for filling
7. Carded on Thick CA (purple bottle)
3.2 gms/ft**2 first coat, thick CA on raw wood – pool a bit on then spread out with credit card
1.6 thick CA – spread out with credit card
Not too bad a surface at all light filling required
8. Carded on Epoxy Finishing Resin (Zap Zpoxy)
3.9 gms/ft**2 first coat on raw wood squeegeed off with credit card
1.6 second coat squeegeed off with credit card
Ready for filling
Some comments:
All of these numbers could be reduced with a little bit of sanding. I waited 12hrs after application before weighing. The dope numbers are probably still decreasing due to VOC.
I think I was wrong about the KBS urethane. It looks like, that after you get enough dope on to build up a surface, that the urethane might actually be just as light or lighter than some of the more standard materials.
Brushing dope on straight from the can is a mess. The stuff is way too thick, you have to thin it a pretty good bit to get it to flow out with a brush. At least the dope I was using (Certified low shrink)
The average coat of dope is about 3 gms per square foot.
Applying carbon veil or 0.5 glass cloth takes a finite volume of dope, epoxy, etc. to fill, even if squeegee is used. About 6-8 gpsf. Some of that is the cloth itself, but most is the material to fill it up.
Brushed on K&B Superpoxy with glass is still a pretty good method to strengthen the wood and pretty well fill the grain. Frank McMillian’s original Martin Baker still weighs only 53 ounces (ready to fly) finished in this manner. (Source: John Hill who has it in his shop)
Squeegeed on Zpoxy finishing resin is pretty easy and probably lighter than straight dope.
Squeegeed on thick CA was a surprise and is really tempting. Gut reaction is that it would be heavy, but it isn’t. Tempting, mighty tempting.
I figure that a 700in**2 stunt ship has about 16 ft**2 of surface area. So … a 10 gpsf weight turns into 160 gms per airplane or about 5.6 ounces per airplane.
Question: do we really need the glass or carbon for strength and rigidity?
Remember, some results may vary and if it lasts more than 4 hours see a doctor.
Topic: KBS urethane surface prep (Read 723 times)
