Lets hear some opinions on this one. How long should you wait before starting to block sand this primer? 2 days? 1 week? The garage is heated, nice & warm. I am eager to get sanding, hate the down time.

The car has minor waves to be leveled out. What is the min time you have waited & still had nice results?

Per PPG tech sheet:
Dry times Between coats 10-15 min
Dry to Sand 2-4 hours

Jason, this very question started a firestorm over at another forum I visit. Listen, as the last post states PPG's own tech sheet says 2-4 hours for "sanding." The problem is, that is after following flawlessly the application instructions on the tech sheet. Most people just don't do that.

Most any guy who is applying primer for the purpose of blocking is going to "push" it a little. They are going to "push" the flash time between coats, "push" the amount applied per coat, "push" the amount of coats applied, etc. They will push it (I mean me as well, it is just human nature) because they want to be SURE there will be enough primer on the car to block it out to perfection. Well, this "pushing" will create trapped solvents in the primer. Two hours is just not going to do it under those conditions. Sure you "could" sand it and paint it, but there is a good chance you will get some die back in the gloss. Sure you "could" block it, but it may shrink up a little after blocking and that flat panel you blocked is wavy again.

The fact is some of us may not know they are "bombing on" the primer and trapping solvents (the jest of the firestorm on the other forum) so waiting longer may be needed. If you do everything by the book you don't need to worry about it (though 2 hours is ridiculas for blocking as far as I am concerned) but most guys just don't know they are "pushing" it. So waiting is a good thing.

Just waiting a week isn't that great of an idea. The top of the primer skims over pretty fast, it also cures first. So you could have trapped solvents under that skim on the top. Waiting a week and blocking still leaves you with a soft primer susceptible to shrinking.

The best way to avoid all problems is to not push the primers so as to not trap solvents of course. But even after doing so wait a number of hours and scuff sand the primer well and leave it another few days or week if you would like. This opens up the top of the primer allowing much more solvents to escape. Waiting is certainly not going to hurt anything.

If you give it a half an hour or hour between coats and put them on thin as you can and make sure your garage temp is good you will still see an increase in primer hardness after it sets a week or so. It is the same with all urethane primers I've ever used. Bottom line is if the stuff sands harder the longer it sets then some curing has taken place in that amount of time. On quality jobs I let it set at least a week before my final blocking, never any shrinkage problems. If you need to get on the primer soon you can apply a heatlamp to the primer after it sets for an hour or so this will speed up the cure time. An electric space heater can also be used if a heat lamp is not available- just keep the panel temp 140ish. The hot summer sun if available can also do this for you free.

Is the garage also heated when you lock up and go inside? If not that urethane will stop its curing process if the temp drops below 55-60 degrees. But then when you expose it back to nice warm temps it starts curing again.
Say you wait 2 days but that primer has only been exposed to 3hrs of the proper curing temps. That primer is dry, but is it really fully cured???

Bob, if I remember right, I dont have the tech sheet right in front of me but PPG suggests not to bake this primer.

As Martin said those tech sheets are written for perfect conditions, one item changes and it ALL changes. I would not wait 3-4hrs like the tech sheet say, even if you applied in those "magic" conditions. Waiting 2-3 days making sure that garage is kept at a temp where the primer can cure is your best bet

Bob, how do you know it is sanding harder? I am just wondering what "standard" you use. I mean, I would have a pretty darn hard time judging how "hard" something sands compared to sanding it a week earlier.

I think I will set up some tests to have panels primed at different times, a week or more apart. Then sand them with some way to know if one is "Harder", maybe with amount of stokes to sand thru or something like that. I'll start that project this week.

Like I have said before, if a primer gets "harder" a few weeks later, something is wrong. It should have been CURED totally CURED long before that.

I am guessing it is not fully cured for 30 days? Paint man. usually tell you to wait 30 days before putting any wax on a car (clear coat) as it is not "fully cured". Maybe primer is the same.
If I prime something on Friday verses baking or sitting over night I can tell that it is harder to sand.

I keep my shop 65+ degrees year around with a heated floor, I still see differences in primer hardness while sanding after the stuff sets for awhile. Martinsr I think you'll see what I mean with your tests, just interesting that you haven't noticed a difference in primer hardness when the stuff sets like I have. I don't use any standard just a heck of a lot more sandpaper, LOL.