Other epoxy primers are good products, I have used another brand for small areas and my painter has used others on full cars inside & out with good results.
SPI is a better quality product overall, especially for the endura bumper guys on firebirds & GTO's, its is much more flexible that standard epoxy primers which is whats needed for those bumpers. Something about it must be different since it has such strict cure times & timeframes for body filler or top coat primer to be applied.
For the average car other brands are fine, just might not look as mice if left uncoated, the one I used was a greenish color & very flat, SPI black is very good looking when left uncoated, looks better than many chassis type paints Ive seen.
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