Can't get silicone caulk to stick to engine bracket.

[RussH]

A year ago this spring I dug out all of the caulking between my engine bracket and transom and refilled the gap with 3M-5200. The 5200 didn't last a month and it separated from both the transom and aluminum engine bracket leaving gaps on both sides of the bead. I proceeded to dig the 5200 out, scrapped with a razor blade, cleaned with acetone, vacuumed, and rinsed with fresh water. I then decided to replace the 5200 with silicone since that was the original filler. The silicone seemed to stick better but twice over the past year it started to separate from the aluminum engine bracket leaving a crack/gap. Today I spent 3 hours digging the silicone out which was made worse because I kept "patching" the cracks/gaps and it got built up pretty high in some places.

I thought I remember someone on the forum that mentioned that 3M-5200 was a polyurethane base and silicone wasn't compatible with it. Could that be the problem? Did the 5200 separate when I applied it over the remnants of silicone, then when I tried to go back to silicone I applied this over the top of the 5200 which is why the silicone didn't stick?

I've dug all the silicone out, scrapped w/ razor blade, wiped down with naptha, sanded with 100 grit sandpaper, and vacuumed. Now what should I do? Does the naptha need to be rinsed off with water or will it simply flash off? A buddy of mine dropped off a tube of Dow Corning 799 silicone that he said costs $35 per tube and is supposed to last for 50 years. Should I use this stuff? What other prep work is necessary? This is way to much work and I want something that is going to last.

Russ

[PeteH]

Is the aluminum bracket oxidizing?

[RussH]

No oxidation, plus if it was it has been sanded twice. It's like there is some kind of residue left from the silicone or 5200. The original silicone lasted for more than 10 years, I was simply trying to do some preventive maint. by replacing 10 year old silicone and ended up creating a problem. The old adage "if it ain't broke, don't fix it" sure comes to mind .

I'm going to try the Dow Corning stuff tomorrow unless someone comes up with another suggestion. I want to use the boat this weekend.

[PeteH]

If you've sanded the powdercoat or anodizing off the aluminum, you will have to prime the bare area to get the sealant to stick for any length of time. 5200 or silicone on bare aluminum will let water weep into the seam from around the edges and cause the seal to fail. Prime with a zinc chromate primer (on a small brush or Q-tip?) and then apply the sealant.

[RussH]

I didn't sand through thru the paint. I guess at this point I will buy some primer and try that. It is a royal pain in the ass to dig out the caulk so I'm willing to try almost anything......except I'm going to put the boat back in the water tomorrow regardless of a solution.

[theoldwizard]

The most common problem with 5200 is not letting it set long enough before using.

7 days.

Yes, read the tube. 7 days