The sealer coat is a unsanded pre coat right before your base coat, it keeps your color from bleeding back into the primer coat underneath, many epoxy primers I have seen say they can be used as a sealer if thinned quite a bit, others like House of Colors products use a product called Ko-Seal for this, This coat is sometimes used to bridge breaks in the primer surface underneath. When I first started painting I would block sand and cut through the primer in some spots, let’s say I have polyester primer on and shot a coat of epoxy on top of that and I had a guide coat on and blocked it and cut through the epoxy and the polyester primer underneath was now visible, you can't shoot base on top of that in this situation as the polyester absorbs color differently than the epoxy and you get a big ol' redo, the layer underneath must not be cut through, so you could use a sealer on top of the epoxy to "bridge" the cut through, sealer isn't generally sanded and is shot before the base, for a pure novice as myself the sealer is worth its weight as that bridge coat, it’s also suppose to make the paint more vibrant as the color sets on top and not soaked in.
The last coat of epoxy I shoot gets guide coated and blocked to a minimum of 600 and 800 if painted black, if you can block a entire car with that fine a grit and it takes off the entire guide coat the things pretty flat, I wouldn't shoot base on the polyester ever as to me its soft and since its dry sanded (sanded in the 400 grit range) it doesn't get smooth enough as a wet sanded epoxy or urethane primer would be and I think it would absorb the color, I haven't ever tried it for those reasons, keep in mind about 90 percent of that polyester ends up on the floor and in my lungs I think (wear a mask).
Like I said I am no expert, the Achilles heel on a paint job for me has been the clear coat, that has been answered by Southern Polyurethane clear coat, my last full paint was the only job I ever have done that I didn't have a issue with the clear coat, not that everyone does maybe just me! I recently had a custom painter tell me the reason I was having such a hard time laying down some of DuPont’s clears and having to color sand them so much was those clears are designed to mimic orange peel in factory paint jobs, as that’s what they are used for nearly all the time, Like I mentioned I had no issues with the SPI product, I wondered if it wasn't all hype but it appears not to be, they cater to custom paint jobs without a paint booth and drying system, that happens to be in my shop!
To me what separates the professional painters and the armatures such as myself is what to do when something goes wrong, I had enough of DuPont’s (awata) tech support (they were not interested in helping a hot rodder) I think HOK's tech is great though but SPI is the best ever, you can call his cell on a Sunday evening and he will answer (which fits armatures very well)
I keep in mind most any quality product is going to be fair on a street rod, I mean how much sun, weather and abuse do they get? I sprayed a vw beetle with Summits cheap single stage paint and daily drove it for 3 years just to see how it held up, and it held up great. I had a guy tell me before you can't take a chance on it to paint a custom rod with it you don't know if it’s any good, so I tried it and it did fine! I know my latest rod will never suffer the abuse I put that paint job through!
Sorry to be so long winded, also Roger I knew what you meant with chalk! Happy Rodding