We recently built several signs that were simple aluminum boxes that were finished with automotive paint and a clear coat. We bought laser cut acrylic lettering from Gemini and used DS tape and dabs of silicone caulk to adhere the letters in place. Several months have gone by since the installation and I got a call from the client yesterday letting me know that one of the letters fell off. When I went back to reinstall and check the other letters, I found that the silicone caulking was not adhering to the paint finish- it is attached fine to the back of the lettering. The silicone did not even create a "ghost" on the paint where it had been applied.
Of course there was very little time between having the cabinets painted and installing the letters. A rush job- what a surprise!!
I'm trying to understand why? Any ideas?
Posted by George Perkins (Member # 156) on :
With there being very little time between the cabinets being painted and you adhering the letters, I would blame it on "outgassing" of the automotive paint. Solvents rising to the surface will cause all kinds of problems, like bubbles in certain vinyls. They could conceivably push off the hardened silicone.
Posted by Bruce Bowers (Member # 892) on :
I was going to say the same thing as George.
I also wonder if you used a fisheye eliminator in the paint that could affect the adhesion factor.
Keep us posted!
Posted by Al Williamson (Member # 2869) on :
Thanks to both of you. I guess the next question is to ask the guy who auto paints for me to ask his supplier about the "wait" time.
Posted by Mike Pipes (Member # 1573) on :
Al,
It's my understanding that when using catalyzed automotive paints, once the paint has hardened completely (24 hours at ambient temp) that's it, there is no further outgassing as it's about as cured as it's going to get. The paint manufacturers even recommend wetsanding and polishing (if needed) within the 24 hour mark because after that, it's much harder to sand.
My bet is the issue is more related to the prep of the auto paint finish itself. Maybe there's some kind of glazing or polishing compounds or waxes that were not completely stripped away, thus affecting the silicone's bond. As I learned when I used to design windows/facades for skyscrapers which use a lot of silicone to bond parts together, silicone often needs meticulous prepwork because it can get quite finicky.
Posted by Ray Rheaume (Member # 3794) on :
Cure times depend on air moisture content, temperature and paint/clearcoat system used.
Warm moving air or using heat lamps helps speed up the drying/curing time, but some auto painters use more or less hardener to speed or slow the cure time, so it can vary from shop to shop.
Additional coats will need additional time to cure as well.
Just as a general rule of thumb, the body shop I work with most and I usually wait at least 3 days before doing anything OVER a fresh clear coat job. Humidity is a major factor most of the time around here and the heat lights are used for at least the first day.
Bruce brings up a good point.. Many body shops have been known to add silicon to the clears as a fish eye reducer. Some of the major manufacturers are as well the last couple of years. It's not usually a problem with an older, fully cured vehicle, but, in my own experience, it can cause adhesion issues on a fresh paint job.
I honestly don't know what kind of effect adding silicon does to the curing time, but hopefully Gavin will chime in on this one.
Hope this helps a bit... Rapid
Posted by Frank Magoo (Member # 3950) on :
Al:
From your decription, it sounds like to me an accumulation of migitgating circumstances is responsible for what is happening to your letters...to explain.
The purported fact that paint is two stage and supposedily "dry" in LESS than 24 hours is simply not true...it still is considered "green" in nature and has a dry period before being considered "cured"...add to that the fact it may have been sanded and polished and if so, not only was original surface "opened", when sanded, it was sealed w/possible contaminants...add all this together; outgassing, contaminantion of gripping surface, and nature of vinyl to "grip" when finally "set"...resulting in "delamination" of offending surface, which of course means somethings going to fall...
Sounds labor intensive, one of many reasons if I was in your shoes, to make sure I KNEW why it had failed before replacing entire sign...if one or two have fallen, shake others and see if they release...might just be a bad batch??? Good luck... Posted by Michael R. Bendel (Member # 5847) on :
Silicone is simply not a very good adhesive period!
There are far better adhesives out there if you choose not to stud mount. PL ADHESIVES for example.
Many 3M & similar exterior tapes will work as well.
It sounds like a very tight pore automotive paint & silicone will certainly fail with ANY expansion.
Use what the automotive companies use to adhere things to automotive finishes. Seems to work well for them? Check with your local AUTO PARTS DISTRIBUTER.
Good luck!
Posted by stein Saether (Member # 430) on :
silicon is nice, if it has a 4 mm thickness, then it adjust to anything
Posted by Joe Crumley (Member # 2307) on :
Let me take a stab at this one!
Since several months have passed, I'd rule out, out gas, as the problem.
I'd guess he used the wrong Silicone. The term "Silicone" doesn't necessarly describe an adhesive. I just wonder what type he really used. An example would be GE Silicone 2. It never dries and has very poor adhesive qualities.
I's use GE VHB.
Posted by Dale Feicke (Member # 767) on :
I'm with Joe on this one. There are many different configurations of silicone. Some are made for sealing out the elements; some are made to be adhesives. They are generally not interchangeable. You need to read the tube. I've glued stuff up with silicone adhesive, that's been up for several years, that you won't pull off with a truck.
That being said, some of the earlier comments about outgassing might also apply.
Posted by Gene Golden (Member # 3934) on :
I never use silicone.
I use "Clear Adhesive" which is paintable also. It is available from Lexel, Polyseamseal, DAP, Red Devil, GE and LiquidNails. There was also one called Universal Clear Adhesive available at one time. I'm not sure if all of them are still available, but I've used them all with good results.
I still find the Lexel and Polyseamseal without problems.
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