Primer adhesion

frainiea · Post by **frainiea** » Sun Feb 23, 2003 8:17 pm

My windshield replacement was moving smoothly at first but not I have a problem. The bottom of the windshield is mounted with a pair of flanges or strips. One mounted to the air frame that is below the edge of the windshield and the second goes over the top and is attached with screws. In my case both had a fair amount of corrosion that I attacked with a drimel and sanding. An Alumiprep 33 then Alodine 1201 according to the directions followed this. So far it looked good. When I started to apply the primer things went wrong fast. The primer, from an aerosol can from my local aviation supply, looked like it was going on a surface that had wax or oil on it. It appears that it is repelled by some of the pits and for an area around the pit with about 1/8 inch radius. I stripped off the primer and repeated the treatment to no avail.

Is it not possible to spray self-etching primer over corrosion products? Is it possible that I did not get all the Alodine properly washed off?

Has anyone had a similar experience with primer?

N170BP · Post by **N170BP** » Mon Feb 24, 2003 3:54 am

My guess is someone in the past sealed the windshield with silicone
and this has worked itself into the corrosion pitting. Find
someone with a large (!) bead blast cabinet and have it bead
blasted, then re-etch & alodine. Speaking of which, use Scotchbrite
with the etch to make sure it works into all of the corrosion pits.

Then, prior to prime, use a quality wax & grease remover to
thoroughly clean/prep the part for painting.

If all the above fails, obtain a cheap "travel" hair drier (kind that
folds up and is compact). Target & Walmart and the like have them
for about 15 bucks. Shoot warm air onto the part with the hair drier
with one hand while spraying the primer with the other. Experiment
with some scrap material until you get the balance of warm air to paint
right. The warm air helps eliminate the fish-eyes on the surface
caused by solvents / silicones (it's an old car detailing trick).

Good luck,

Bela P. Havasreti
'54 C-170B N170BP

Dave Clark · Post by **Dave Clark** » Mon Feb 24, 2003 12:38 pm

Be very careful using any media blast. It needs to be done at low pressures and great care to avoid heat distortion that will ruin the part. Try cleaning more using regular paint thinner. Soak it and it will attack the silicone and ultimately dissolve it. It will be hard to see if you've gotten it so it could be a trial and error for a while. Before priming wash off with a lot of fresh paint thinner and then use MEK or Acetone as a final wash rotating your clean paper towel frequently. Scotchbrite and wire toothbrushes are your friends here not the Dremel. You might not get it all.

The propellant for paint cans is highly flammable so be careful with the heat gun.

Had a friend who set up a paint can some distance from an electric heater in his hangar to warm it up prior to painting and went inside to get a cup of coffee. He heard a noise and went out to the hangar to find the paint can exploded and the tail of his Airmaster on fire.

frainiea · Post by **frainiea** » Mon Feb 24, 2003 12:39 pm

The old windshield was indeed put in with silicone sealer. Thanks for the input.

Post by **GAHorn** » Mon Feb 24, 2003 2:28 pm

Some silicone sealants produce ammonia while they cure and contribute to corrosion. The windshields should be installed with felt and be allowed to "float" in the frames to prevent cracking while taxiing. (Watch your windshield when you taxi on a rough grass strip and you'll be amazed at how much it moves within the frame.)