SEM Rust shield failure

[drcoastline]

As the title implies the SEM rust shield on my gas tank is failing when top coated with Upol Gravitex. This has now happened twice in the same spot.

After I made modification to my tank I mixed the SEM 8-1 with hardener as directed and sprayed on several medium coats for protection. This sat for many months before I was able to return to the tank. Last week just prior to top coating with Upol Gravitex I noticed a crack in the filler I used to blend the welded piece into he surrounding tank. I cleaned out the filler. recoated the area with SEM, applied filler again and blended. I applied a second coat of SEM the following day. I allowed the SEM to flash then attempted to apply the Upol as a wet on wet coat. It crazed with in a minute and ran off in waves where I applied the new SEM. I scrapped off the clumps of Upol and allowed the rest to harden before sanding. Yesterday I applied new SEM rust shield 8-1 to the area and allowed to harden overnight. I went out this morning and scuffed the SEM and previously applied Upol, cleaned off the dust, wiped down the surface with RM 901 and began applying the Upol. It was going on perfect. I sprayed a light coat let tack sprayed on another light coat. I did this three or four times to build up a nice coating with out causing any runs. On my fourth or fifth time back I noticed the Upol was starting to crazy. Within seconds it ran off the tank. This time I noticed black in the Upol. The SEM was wet. It seems the Upol is dissolving the SEM. But only in this area that is newer SEM. I can only assume the SEM didn't fully harden.

[962porsche]

the 1st thing is rust shield is a one coat product . when you over apply it the under layers do not sure they will dry but not cure .
the max mil thickness is 3 mils and no more then that of the rust shield . from the looks of your finger imprint it looks like you have 7 or 8 mils at the least on there .
after you fixed what was wrong and reapplied the sem it's a 7 day wait with the metal temp of 70 degrees . if the air temp is 70 in your shop it doesn't make the metal temp 70 it would be more like 60 for a metal temp this you would have to add at least another 2 days for full cure time .

because your pushing the cure times to much your making it solvent sensitive .
when your doing chip guard flask time is very important also .
they say it 10 to 15 minutes from your 1st coat to your 2nd coat if you trying to build up the chip guard and say are doing 3 coats you will need to add more time for flash off from the 2nd to the 3rd coat so your flash time would be more like 30 to 40 minutes .
but again that is for 70 degree temps . and again it's never air temp they are talking about it's metal temp of the thing your spraying .
now this is when your shops temp and what you do for heating your shop comes into play .
if your not heating the shop when not in use or your turning the heat down so far it cold in the shop when your not in it then this adds more to how you need to do things like painting .
just because you turned up the heat in the shop and your toasty when your in there the metal is still cold .
what this does is either just make the part to cold to spray if the metal temp is under 60 degrees or you will have to wait until the shop gets and stays worm enough to get the metal temp over the 60 degree mark it has to be to spray .

after you have spray your coatings and before the part is dried all the way and you leave the shop then just turn the heat off or down real low and the metal temp drops below the 60 degree mark the same thing can happen . it could also delaminate because the last coat that was sprayed dried faster then the earlier coats that were sprayed . so the top coat would be dry and the under coats would still be wet . because of this the solvents from the under coats can not escape thru the dry and harder top coat . the solvents being trapped is like filling a balloon with air to much and it has to escape some were so the balloon pops . this is kind of what your seeing the wet solvents have to go some place and it popped / split the hard shell of the top coat of chip guard .

the problem i'm seeing your going to have to to clean the tank back down to bare .
my guess is every thing under the chip guard is wet and will stay that way for a long long time .
it can't cure with out air getting to it .

[962porsche]

i just did the trunk of this car last week .
monday i sprayed 2 medium coats of mixed 8 to 1 gray rust shield .
friday i scuffed it with a red pad and sprayed two coats of gray gravitex with 20 munites of flash between coats .
i then waited 2 hours and then sprayed the two part/2K splatter coating applying 3 medium coats with 15 minutes from the 1st to 2nd coat and 30 minutes between the 2nd and 3rd coats .

the important this is if you can't touch it ... in most cases it's not ready yet for the next coat .
the only thing this (doesn't) apply to is single stage top coats and clear coats . with them you want to leave a finger print in the surface but nothing comes off on your finger .

with my shop it's never cold ! NEVER !!!!!
my daily shop temp is in the mid to high 70's and nights it's set at 70 degrees .
warmer then many houses are but it has to be to keep the cars metal temps at the 70 degree make .

[drcoastline]

Everything you said I did wrong on this area. The SEM is probably thicker than 3 mil. I did mix 8-1 but did not wait seven days. My shop is heated but I did not wait long enough for metal to come up to temp. It was 65 degrees outside yesterday but I doubt the metal surface even with heat was 65-70. I also was recoating the Gravitex at about 15-20 minute intervals. Well clean it off is what I will do. Better safe than sorry.

Thanks 962.

[962porsche]

if you have one of the laser temp gun check the temp of the metal with it .
like i always say read your tds ! the flash and dry times i gave are just off the top of my head and i could be wrong .
i always say when it comes to applying products more is not better .
so if they say one coat and no more then 3 mils that is all that is needed for protection of the sudstrate .
even if you didn't go over the sem with the chip guard you could run into problems with lifting pealing and delamination down the road .
so if you do something like say a chassis /pan you still have to go by the mil build the TDS gives you .
i know all to well when spraying something like a fuel tank it has nooks and crannies hills and valleys so trying to only spray one coat and getting it to look good is a PITA .
i have found that when i have something with lots of tight places i need to get coverage on i will just hit those areas 1st with a coat left those areas flash then spray the hole tank/part kind of disregarding trying to get more sem on the areas i spotted in 1st .
if i remember right sem wants you to use a 1.6 to 1.8 setup for your gun . tip sizes kind of tell you how the product builds . being say 1.6 to 1.8 your dealing with a medium to high build product . products like this are some times more flash sensitive needing longer flash times then what they may state in the TDS .
the TDS is written for the ideal conditions . if your not spraying in a heat and humidity controlled booth you will have to adjust your spraying to deal with your conditions .
one way to deal with it is to hive longer and longer flash times between coats . don't think this is a fix for it just being way to cold to paint but in some cases it will work .

[962porsche]

this should give you some idea of the temp differences.... i got this for temp when i opened up the shop today .
shop temp is 68 degrees and the metal temp is 61 degrees .
with the out door temp being 49 degrees .

[drcoastline]

Yep, I'm going to strip the tank and try again. As always thanks for the advice. I'll keep you posted.