Stress Cracks

[TCS4605R]

I've DA sanded all of the color, primer, etc. of the main body shell and just finished the same on the bonnet.  I've discovered one thing that I did not expect regarding stress cracks, spider cracks, etc. whatever you want to call them and I had some cracks that were as wide or wider than your finger nail.  The cracks do not (at least my cracks) originate in the fiberglass body but in the sprayed on finish.  On the main body shell, the cracks were just in the color for the most part.  On the bonnet (the bonnet had primer + epoxy sealer + color coat), all of the cracks were in the color coat and the epoxy sealer, but not in the primer or the fiberglass.

I've had lengthy conversations with my local PPG rep and done a lot of research on the Internet - they all say to let the high build primer/filler sit for 4 to 6 weeks before applying any further coatings.  This is after you have block sanded the body down and are happy with the flatness of the surface.  This lets the primer fully cure and hopefully do all of the shrinking that is going to take place.  If you can let the body sit out in the sun, all the better, but the sun thing is not possible for me this time of year.

I have considered applying waterborne color and clear coats, but was basically talked out of it by the PPG ref.  First of all, the PPG folks will not sell to a 'guy that works in his home garage'.  I think that corporate PPG requires them to sell to legitimate body shops and not guys like me.  Second, the drying time for waterborne is much longer that solvent paints.  Not having a spray booth (or anything that resembles a spray booth), the longer drying time for waterborne is just that much longer for bugs, dust and general crap to come to rest in the wet paint.

I have decided on PPG 'Deltron' single stage urethane paint.  They can exactly match the Lotus '07 Yellow' color.  PPG also tells me that, if I prefer and want a deeper color, I can apply clear over the single stage urethane.  I will also use all PPG products under the color coat so I can be assured of compatibility, but I kind of think the compatibility thing is a bit of sales hype, but why take chances at this point.

Anyway, these are my thoughts on 'stress' cracks.  The PPG rep did say that todays product and color coats are much softer than yesteryears lacquers, enamels, etc. thus they are able to comply with flexing fiberglass more easily.  Anyone have any thoughts of the 'stress crack' dilemma?

[buzzer]

Interesting if your cracks are just in the paint layer. This is unusual, are you sure. When I did my body I took off all the paint. Then sanded down the gel coat where I had cracks. I did find it very difficult to see the cracks when I did this. I wiped on some dye with acetone and this showed up the cracks in the gel coat so I knew where to take it back to.

 On specialist primers have you looked at UPOL reface . A polyester spayable filler. I used this to get my surfaces level


Dave

[TCS4605R]

Dave,

I found no evidence of gelcoat on any part of the car.  If it was there, it was a coat of clear resin.  After sanding off the paint and primer, I wiped the surface with acetone colored with red 'Mont Blanc' fountain pen ink - not cracks in the fiberglass were visible.

I plan on using PPG MP282 High Build 2K Primer Surfacer mixed with PPG MH283 Normal Temp High Build 2K Primer Hardener - this is a sprayable high build primer.  I will then block sand this and recoat as necessary until I'm satisfied with the surface, then let the primer cure for 4 to 6 weeks.  I need to talk with the PPG rep again - I may apply a coat of PPG MP172 Black Epoxy Primer to the bare fiberglass first and then a coat over the MH282 primer after the 4 to 6 weeks primer curing time, but I'll rely on the PPG for guidance there.

I'm going to finish the bonnet first to see how it works out and if it does not, I'll get the DA sander out again and start over.

Tom

[EuropaTC]

Hi Tom,

I'm slightly surprised that your PPG rep won't sell water based paints to you, unless he means that they won't sell because they direct all their products through main agents ? Usually it's the isocyanate based things that draw the dark looks when you admit to being a hobbyist without a spray booth.

However, back to cracks. Those such as you describe - finger nail wide - do sound very like cracks I've seen which are due to either the thickness of paint coat or reaction between two different paint systems when someone has sprayed over an existing finish. Flatting down to the gel coat will remove all of these. But it is unusual not to find any gel coat cracks. Even if the bodyshell hasn't suffered any accident damage over it's life there are usually faint cracks around the mounting points such as the windscreen wipers and front bonnet lock areas. (at least there were on mine)

It's hard to argue about the length of time you plan on leaving the primer, but it did surprise me. In the days of cellulose or solvent evaporation paints then yep, that was the sort of thing the better paint shops did. But 2-pack, chemical cured paints were supposed to speed up the process, cure much faster for turn round in accident repair shops so it did surprise me that the PPG rep is advising that course of action.

Finally, another vote for sprayable polyester filler (eg Upol, there are other brands as well). It is much different from primer fillers of any variety and will make the job far easier. IIRC it was also used by Lotus when new to speed up production times.  The downside is you need a gravity fed gun with a large nozzle (2.2 on mine ?) and a good air supply. But it's well worth the trouble, especially if you're spraying over repaired areas.

Brian

[Chuck Nukem]

I had always assumed that gel coat was used as the base layer on these cars. I had substantial cracking on mine and you can definitely see it in the fiberglass itself. I will give the Upol stuff a shot, it sounds like a winner. Brian, would you spray that directly on to the fiberglass or seal the car first with an epoxy primer?

[EuropaTC]

I will give the Upol stuff a shot, it sounds like a winner. Brian, would you spray that directly on to the fiberglass or seal the car first with an epoxy primer?

Whether you go for Upol or some other version, yes, as far as I'm concerned it's the best thing since sliced bread. 

It's polyester based so what I tend to do is repair any cracks using resin/tissue and flat off with 80 grit production paper, keeping everything dry.  Once the surface is contoured I'll often use 120grit to take out any deep scores but after that I spray directly on to the fibreglass with no other sealing coat.  It has good adherence and I leave it overnight before taking off the worst of the orange peel the next day with 240grit used dry.  If I break through I'll re-coat the panel, the aim is for a complete covering all over the car. (that's just me, most likely you don't need that).

I've never sprayed over paintwork although it has such good hold-out and covering power that I'm sure you could. But following that procedure has worked over repaired areas very well, whereas in the past using just cellulose fillers I would get sinkage a few months down the line.  (hence the 6-8 week drying time comments ?)

Brian 

[Chuck Nukem]

That sounds great! There is no paintwork left to spray it on to currently. I am stuck in the wretched job of hand sanding all the little crevices.  It seems to take almost as long as the sanding of the entire car! I still need to grind out and build up the spider cracks, but I am hoping to have this thing in primer next month!