This was posted by a member here (PaulatFast) who was always posting great information, too bad he is gone, as I always read his posts to learn something,
Hope this helps
To sum up the system we use, I submit the following. Brands are named, not to aid those brands in sales, but just for reference because they work for us.
First we sand blast the endura bumper, not soda blast, not sand with a DA...
I have found that ANY brand of flexible epoxy repair product seems to adhere well, sand easily and last as long we've had them out there. 3M, Medallian, some french canadian product we have right now... same smell, working time, etc.
After repairing the issues we fit the bumper to the car.
This procedure involves bending, twisting, jumping on and sometimes beating the bumper with a5 lb sledge hammer. It involves trimming and/or building the edges with the 2 part epoxy. The bumper brackets and the frame horns usually need the slots welded up and ground back down, smetimes the lower front of the frame horn needs trimmed to allow the bumper to go back far enough to achieve nice gaps. Remember, the factory gaps sucked most of the time, as evidenced by the GM photo's from back in the day. What is a nice gap? 1/16" to 1/8" on a 68-9, and 1/32" to 3/32" on a 70. NO TOUCHING!
After the nose fit is perfect, or good enough for you, we spray several coats of SPI Epoxy primer, it sands easily, sticks to the endura and the epoxy repair material. This is where I feel brands are inportant. Some epoxy primers simply do not sand well, ever! The SPI does, so we use it! Others may work, I don't know. We use no other primer, no 2k, nothing but the SPI epoxy. We recoat and block as needed. We sand the epoxy through 600.
We use the same SPI epoxy and sealer before basecoat or single stage. We have used flex agent in the base in the past, never caused an issue. We don't any longer, after researching what the flex agents actually are... simply retarding agents that prevent fully curing.
We have sprayed many bases without issue, Nason, Omni, Dupont Chroma-Base, PPG DBC, Pro-Spray, Zenith and our current mix station, PPG Global. or the record, avoid Napa, Martin Senior CrossFire on anything. Horror stories too numerous for this thread...
We spray SPI Universal High Solids Clear, it sands and buffs well, doesn't yellow and sticks to anyone's base. The following is totally my observation and opinion. Good, Quality, High solids Clears don't harden and become brittle like cheap clears do. We have sprayed Dupont 7900 in the past, it is pretty good but i've repaired noses that were sprayed in 7900 14 years earlier, and the clear was very brittle. That said, there are probably numerous clears that meet this criteria. I use what is proven and works for me.
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Paul
FAST Automotive -The Pontiac restoration shop, not the fuel injection guys! I had the name first.
My site... needs updated-
www.fastrestorations.com