high build primer

I would suggest you pickup some primer that is available in aerosol cans. It is available at most autosupply stores.

i don't know about the brushing, but the autozone type can hi-build primer is only good for tiny corrections.  I have use the bondo spot putty and it works a lil better.  Put is on in very thin layers and let dry completely.

I am new to this forum however i have been in car stereo industry for about 14 years. i now currently do restorations  and hot rods but still do alot of fiberglassing. we all know the purpose of high build primers. and honestly, as much product as i have come across. nothing out of a spray can is truly high build. some of the stuff even reacts with base and clear. the atomization process of a spray gun is the only tru way to get the build needed to fill and keep a consistant coat. if your fiberglass needs no body work, u can go straight to prime. if u have to do body work. use the correct papers and fillers. my pet peeve is awsome fiberglass with horrid finishes and bad body work that could have bene perfect if someone took their time. 90% of bad body work is resulting from using the wrong stuff.  first anything being covered with a filler needs to be sanded with at the minimum 80 grit for it to stick. dyna glass( fiberglass reinforced bondo) is for thick repairs. it gets sanded with 36,  bondo is for smaller fills and should not exceded an 1/8th inch. it can also be sanded with 36 to make quick work of it. i prefer rage gold, its great stuff  sands really easy and its cheap. all your pin holes and  minor inperfections get some putty, never use the stuff that doesnt take hardener it will shrink. i sand this down with 80 or 180.  now your ready to prime.

on raw fiberglass that has been knocked down with some 80grit i have had good luck using evercoat slick sand. we call its spray bondo cuz it will fill craters. it is made for bare fiberglass, has excellent adhesion properties and when sprayed out of a 1.8 or even 2.2  tip will build very well.  u can pick up a cheap gravity feed gun anywhere these days.  i say gravity feed becuase it is the most efficent gun u can use. hvlps waste no material and they are very easy to clean and lets face it, anything thats to complicated to clean usually gets left for dead after a while. your idea of using a brush or roller would work, but it is creating alot more work then u think. the gun lays out the product evenly and smooth. a brush will leave brush marks and thick spots that when it becomes time to do your block sanding will create more work  and possibly not as nice of a final finish.  2 or 3 good high build coats. spray a guide coat over it, , block it down with some 80, and then 180 over it to knock down the 80 scratches.   if everything looks good, id hit it with one light coat of primer again to fill the previous scratches, do a quick block with 180 then  sand it with 320,  wetsand with 600, seal it, paint it. if theres trash , runs or other inperfections in it. then wetsand and buff it.  these are the same processes we use on $10,000 paint jobs that leave our shop every day. hope this has been informative and helped at least one of you out there evne if it did go off subject some. good luck with your projects.

Couple things to consider....HS aerosol primers are NOT the same as HS 2K primers. Pick up a technical data sheet and look at the "solids" content, and this will give a good indication if you are using a HS primer. Can you brush this...of course why not? Is that what the High priced chemist' spent hours formulating that product to do? No. The down side is that you may not get the fill or sandability you desire in the same amount of coats.

Aside from the aerosol "high build" (using the term loosely) There are other products on the market....PPG has a line called "specialty coatings" They HAVE formulated a rollable 2K primer. Use this with a 3" foam roller and you are good to go! ( some things to know before using this, 1. It is not recommended on flexible parts. 2. It does not sand very well.)