MG MGB and MGB GT Tech Talk
I found a little surface rust in my trunk, so I sanded all the paint off and coated the floor with POR-15. Then I covered that with the tie coat primer that POR sells. I let it dry 24 hours. Now I'm trying to paint over that with acrylic enamel, but I get an alligator finish where the paint is lifting. What am I missing?
When I had mine sprayed they painted right over the POR, but it was also textured (under the wheel wells).
Did you let the POR dry for at least 24 hours? That might be the problem. It has a tendancy to shrink a little (ever see a can that's been left open? it starts to implode!)
Looks like monster solvent pop to me. Like something about some part of the POR 15 hasn't completely cured and the solvent vapors are lifting the paint. Sometimes I wish I had never mentioned POR 15 on this site. The only thing I would ever consider it for is their floorboard repair and their gas tank sealer. I used the tank sealer on my car, but otherwise had only a little surface rust in the floors and trunk. Any primer that bonds well to metal is good enough for that.
POR drying time is inversely related to humidity. Was the tie-coat primer put on thick or just dusted on?
I brushed the tie coat on, so it was pretty thick. The tie coat is mostly off now. I used lacquer thinner to remove the alligatored paint and it also took most of the tie coat off, so I'm back to the drawing board. Should I try another couple of coats of tie coat and them let it dry a few days?
I have found the SEm product to work well; coat, dry, cover with primer/sealer, then shoot. No problems.
Hey John! Don't wish that! The products you have mentioned here in the past few years have been some of the best stuff I've seen. When I did my 66 shell I painted the entire underside, floor boards, inside sills, trunk and wheel wells. The paint hasn't come off yet, so it must have something to do with the curing time.
It just makes me feel bad to see someone having problems with something I recommend. I'm sure the stuff wasn't cured before he painted it and that was the problem. I won't even put a color coat over over primer for at least a week. Solvent lift is bad enough, but solvent pop the first hot days of the year is a real heartbreaker.
I don't blame you John or anyone else(with the possible exception of POR-15 whose instructions I followed to the letter).
I have used POR-15 a couple times - MGB gas tank and on my Accord's windshield channel (rusted bad under windshield). The thing I have noticed is that it seems to take quite a bit longer to dry than advertised. Maybe it was the weather (in garage - during winter) that delayed it. But the gas tank took what seemed like forever to dry rock hard.
I also use the POR Patch to seal some larger rust holes in the windshield channel and POR 15 in the rest of the channel- worked great. I let the car sit a week after applying before I called the glass co back out to install the windshield. I just wanted to make sure there would be no contamination of the glass adhesive and the POR.
Paul S
Without looking I'm sure the instructions call for priming the POR 6-8 hours into its drying time when it still has a slight tack to it.
POR drys quicker when the humidity is higher rather than lower.
Al
The tie-coat primer says it can be applied over POR-15 upto a year later.
from POR-15's website :
TO TOPCOAT A FULLY CURED POR-15 SURFACE:
Wet sand with 600 grit until gloss is dull, then paint, or use POR-15 Tie-Coat Primer directly on the cured POR-15 surface, then topcoat as desired. NOTE: Never topcoat a POR-15 surface that hasn't dried, as this will ruin the coat already down.
PRIMING OVER POR-15: Use POR-15 Tie-Coat Primer after POR-15 is dry, following directions on can. Tie-Coat Primer may be sprayed or brushed in multiple coats if desired. Sands to a perfect finish.
I don't know where you live, but if it was too cold, the problem could just be the tempurature. Cold material, or cold surface will alligator bad.