First let me address something I see pop up often about paint,but it applies to primer too-DON'T mix lacquer with urethane!Lacquer will wrinkle urethanes and enamels,and urethane is too hot to put over lacquer,adhesion probs will happen most of the time.I have little faith in the urethane clears 'made' to go over lacquer.They work to a point,but it just ain't right.
Now to use primer.I use PPG so I'll refer to it.If you have good clean steel and you don't need bodywork,a coat of green epoxy primer(DP40) is all you need.Let dry,scuff and seal.A wash or etch primer first is a good idea,but I seldom use them.If you need to smooth out the surface you'll need a surfacing primer.I do all my priming and bodywork on top of DP because it gives the surfacer,bondo or putty better adhesion than bare steel.For that matter,they stick better to sanded paint than steel.Finishing the bodywork off in 150 or 180 is fine,anything finer is pretty much wasting time.Remember a few things about primers that are for filling.First read the directions and pick the proper reducer,don't use a faster reducer to cut down dry times.The purpose of it is to fill sanding scratches and feather edges to level the surface.I see alot of sand scatch shrinkage in paint jobs and 99% can be traced to the primer.The most common cause is using too fast a reducer,hosing on each coat,or pushing the flash and sanding time.Your primer needs to be put on thin but wet,not dry.Shrinking happens because the primer is bridging the scratches and not sinking to the bottom,or it was sanded too early and is still releasing solvents so it is still shrinking to it's cured state.I always reduce my primer whether it's an option or not.If I'm filling anything finer than 240 grit I add a touch more reducer and let each coat flash longer.Gun setup is important too.The finer the gun breaks it up the less chance you have of bridging or shrinking.On the first prime use 3 coats.If I finished blocking with 220 or finer,sometimes I'll only use 2 coats to reprime,but usually 3.On my final sand I want 1 coat left on the thinnest spot.A spot of bare steel isn't bad,but a spot of bondo or putty will suck up the sealer and sometimes leave a dull spot in the topcoat.You wont go wrong going 3 coats per prime,and only 1 is wasting time and material unless it's a scuff spot to cover up the bondo spot above.Dry times are another quirk of mine.Many say you can sand in an hour to a few.I learned in the lacquer days to let the stuff dry,so I like to wait until the next day to block or final sand it.My dry times are usually based on the product's window,or recoat time before it must be sanded and reapplied(in general terms).Take DP for example.It has a window of 1 or 3 days,depending on the hardner used.If it stays soft for 3 days and you use it for sealer and paint it the same day,as it cures it sucks up the topcoats and dies back.If you wait till it's dry,sand and paint it's a much nicer finish.I will tell you don't ever sandblast steel and DP and paint it the same day.The final finish looks like it was blasted and you have to let it cure out and sand and buff it.The real reason to scuff and buff is to knock out dirt nibs(or runs) or level out peel,not sand out a shrunk up finish.There's more to it,but this should keep you out of some trouble for now.
Now to use primer.I use PPG so I'll refer to it.If you have good clean steel and you don't need bodywork,a coat of green epoxy primer(DP40) is all you need.Let dry,scuff and seal.A wash or etch primer first is a good idea,but I seldom use them.If you need to smooth out the surface you'll need a surfacing primer.I do all my priming and bodywork on top of DP because it gives the surfacer,bondo or putty better adhesion than bare steel.For that matter,they stick better to sanded paint than steel.Finishing the bodywork off in 150 or 180 is fine,anything finer is pretty much wasting time.Remember a few things about primers that are for filling.First read the directions and pick the proper reducer,don't use a faster reducer to cut down dry times.The purpose of it is to fill sanding scratches and feather edges to level the surface.I see alot of sand scatch shrinkage in paint jobs and 99% can be traced to the primer.The most common cause is using too fast a reducer,hosing on each coat,or pushing the flash and sanding time.Your primer needs to be put on thin but wet,not dry.Shrinking happens because the primer is bridging the scratches and not sinking to the bottom,or it was sanded too early and is still releasing solvents so it is still shrinking to it's cured state.I always reduce my primer whether it's an option or not.If I'm filling anything finer than 240 grit I add a touch more reducer and let each coat flash longer.Gun setup is important too.The finer the gun breaks it up the less chance you have of bridging or shrinking.On the first prime use 3 coats.If I finished blocking with 220 or finer,sometimes I'll only use 2 coats to reprime,but usually 3.On my final sand I want 1 coat left on the thinnest spot.A spot of bare steel isn't bad,but a spot of bondo or putty will suck up the sealer and sometimes leave a dull spot in the topcoat.You wont go wrong going 3 coats per prime,and only 1 is wasting time and material unless it's a scuff spot to cover up the bondo spot above.Dry times are another quirk of mine.Many say you can sand in an hour to a few.I learned in the lacquer days to let the stuff dry,so I like to wait until the next day to block or final sand it.My dry times are usually based on the product's window,or recoat time before it must be sanded and reapplied(in general terms).Take DP for example.It has a window of 1 or 3 days,depending on the hardner used.If it stays soft for 3 days and you use it for sealer and paint it the same day,as it cures it sucks up the topcoats and dies back.If you wait till it's dry,sand and paint it's a much nicer finish.I will tell you don't ever sandblast steel and DP and paint it the same day.The final finish looks like it was blasted and you have to let it cure out and sand and buff it.The real reason to scuff and buff is to knock out dirt nibs(or runs) or level out peel,not sand out a shrunk up finish.There's more to it,but this should keep you out of some trouble for now.