[JSPONT]

maybe in some places it does not matter, I live on an Island surrounded by salt water and humidity. Bondo is porous so direct over metal that can cool and heat and make moisture, then work its way thru the bondo. Now you have bubbles. You should put the epoxy down 1st. You guys in the dry states are lucky but I would still do bondo over epoxy.

[61-63]

A fellow on the Autobody forums did a test about ten years (it may have been fifteen years) ago on two pieces of sheet metal after thoroughly cleaning/degreasing both sheets. He scratched both sheets with sand paper and then on one he shot epoxy primer and then applied bondo, while on the other he applied the bondo to the bare metal. After several days of drying time he bent and twisted both sheets to find out which had better adhesion. The bondo popped loose from the epoxy primed sheet more readily than from the bare metal sheet. This was at the bends and twists. But it stuck pretty good to both and if you ever got in a wreck that twisted your sheet metal to the point the test pieces went through in the fellows test, your car would be so far gone you
wouldn't be concerning yourself with whether or not you should have gone the other way on the bondo/primer thing.

[61-63]

Originally Posted by JSPONT

maybe in some places it does not matter, I live on an Island surrounded by salt water and humidity. Bondo is porous so direct over metal that can cool and heat and make moisture, then work its way thru the bondo. Now you have bubbles. You should put the epoxy down 1st. You guys in the dry states are lucky but I would still do bondo over epoxy.

Your comment assumes you never put anything on top of the bondo. If you shoot epoxy and then paint on top of it the water will never have a chance to get to it.

[JSPONT]

Originally Posted by 61-63

Your comment assumes you never put anything on top of the bondo. If you shoot epoxy and then paint on top of it the water will never have a chance to get to it.

No the bondo is sandwiched. After the body work, the car gets primed (high build multiple coats, then the fun begins, block sanding for days. After that, a coat of sealer, the base coat over the sealer (no sanding if you can paint right away) then clear coat.
The new bondo out is much better than old. The stuff I use is $47.00 per gallon. Bet that test might fair different with the newer materials.
If I leave a stripped piece of metal out overnight, there can be surface rust by morning in the summers here. Humidity is killer to bare metal.

[mrrat1]

What happened to using metal prep on bare steel before primer?

[MUSLCAH]

Originally Posted by mrrat1

What happened to using metal prep on bare steel before primer?

Self etching primers eliminate that step...the need for speed.

[MUSLCAH]

I have never in 40 years in the Autobody Business been told to put body filler over any primer period. And I have never had a customer come back with any problems....

[nytrainer]

Originally Posted by MUSLCAH

I have never in 40 years in the Autobody Business been told to put body filler over any primer period. And I have never had a customer come back with any problems....

Most people don't keep their cars long enough to see problems and even some hack work can usually last past a usual guarantee period . But I'd bet if we had epoxy primer back in the 70's more of the old cars would still be around and in much better shape bodywise. I've been at it roughly the same amount of time (1978) and if I didn't follow the trade technology changes I'd still be shooting lacquer primer over metal prep and then lacquer color/clear or Centari acrylic enamel w/hardener over that lol. No one is saying that you can't put body filler over metal but today filler over epoxy is far better in my book. Feathers like a dream too.
When you stop and think about how the trade has evolved over the last 40 years it's surprising. In addition to changes to the paint materials used I remember brazing patch panels on/or gas welding panels using coat hangers for filler rods. Now there's affordable mig/tig and even panel adhesives. We used paper masks or single/double cartridge half masks now there's affordable (and mandatory for me) fresh air supplied hoods and half/full masks. I loved my Binks 7 guns back in the day and hated HVLP gravity fed guns. Now they're all that is used today for primers and color/clear. Stud welders versus drilling holes and pulling dents (most never welded the holes closed). Even the grunt work of wet sanding is easier today with short throw D/A sanders using 3000 grit or finer. Not sure I'll ever like water borne paint systems though, unless I'm forced to I guess.
For those who aren't really up on primers don't confuse self etching primer with epoxy primer. DON'T put filler over self etching primer unless you enjoy watching it peel off when you blow off the panel lol. If you use self etching primer (some shops who media blast cars or frames will use this) just be sure to remove it where you use filler.

[MUSLCAH]

Messed with Snowball filler when I was 17... Didn't know much about the whole process..but enjoyed it. Then at age 20 in 1976 a guy that owned a local corvette shop asked me if I would like to make more money than working at the hardware store...I said heck yeah ! Wasn't allowed to use a DA or an air Board...all hand sanding and just a cheese grater and the hand long board and short board.....brazing...or gas welding... 600 grit was the finest paper .....and we would wet sand with 600 and buff ..YIKES...but it worked ! Sprayed XIM....Duclos (sp) straight enamel from DuPont ..oh and the "Factory Pack" lacquer too..lol. Ditzler,RM supermax, Centari, Sherwin Williams, PPG, the one with the Parrot on it, And sprayed them all with the # 30 tip on a Devillebiss siphon gun....lol then in 1981 the MIG Came out !!!!......Then Gravity Guns...And structural Adhesive...light weight bondo...5000 grit sandpaper....plastic car cover instead of bed sheets....boy things have changed.....in 40 years..LOL

[JSPONT]

Originally Posted by nytrainer

Most people don't keep their cars long enough to see problems and even some hack work can usually last past a usual guarantee period . But I'd bet if we had epoxy primer back in the 70's more of the old cars would still be around and in much better shape bodywise. I've been at it roughly the same amount of time (1978) and if I didn't follow the trade technology changes I'd still be shooting lacquer primer over metal prep and then lacquer color/clear or Centari acrylic enamel w/hardener over that lol. No one is saying that you can't put body filler over metal but today filler over epoxy is far better in my book. Feathers like a dream too.
When you stop and think about how the trade has evolved over the last 40 years it's surprising. In addition to changes to the paint materials used I remember brazing patch panels on/or gas welding panels using coat hangers for filler rods. Now there's affordable mig/tig and even panel adhesives. We used paper masks or single/double cartridge half masks now there's affordable (and mandatory for me) fresh air supplied hoods and half/full masks. I loved my Binks 7 guns back in the day and hated HVLP gravity fed guns. Now they're all that is used today for primers and color/clear. Stud welders versus drilling holes and pulling dents (most never welded the holes closed). Even the grunt work of wet sanding is easier today with short throw D/A sanders using 3000 grit or finer. Not sure I'll ever like water borne paint systems though, unless I'm forced to I guess.
For those who aren't really up on primers don't confuse self etching primer with epoxy primer. DON'T put filler over self etching primer unless you enjoy watching it peel off when you blow off the panel lol. If you use self etching primer (some shops who media blast cars or frames will use this) just be sure to remove it where you use filler.

True except for the waterbase, LOL I like waterbase, the metallics look way better, but there is good and bad points. Where do you work on the Island, I am sure I know the shop. I'll have to stop in and say hello.

[nytrainer]

Originally Posted by JSPONT

True except for the waterbase, LOL I like waterbase, the metallics look way better, but there is good and bad points. Where do you work on the Island, I am sure I know the shop. I'll have to stop in and say hello.

Yeah I fought the advent of Urethane paints when they first came out only to find out their superiority to lacquers, acrylic enamels, etc. I guess I'd eventually come around to waterborne. Work? I've been in the game too long to still be working lol. Managing, financials is enough work but once you have solvents in your blood, so to speak, you always find yourself still working on projects be it cars, bikes, guitars, etc. I don't have to but I still do lol. I enjoy it (in moderation).

[JSPONT]

Originally Posted by nytrainer

Yeah I fought the advent of Urethane paints when they first came out only to find out their superiority to lacquers, acrylic enamels, etc. I guess I'd eventually come around to waterborne. Work? I've been in the game too long to still be working lol. Managing, financials is enough work but once you have solvents in your blood, so to speak, you always find yourself still working on projects be it cars, bikes, guitars, etc. I don't have to but I still do lol. I enjoy it (in moderation).

Lucky you!! Enjoy, I want to hit lotto so I can retire and mess with old junk all day!

[geeteeohguy]

I used body filler over bare metal 31 years ago on my '65 when prepping for the paint job is still has. Car was a daily driver and not garaged until about 15 years ago. So, 15 years parked on the street, in the weather. Car shows no signs of any paint or filler lifting issues. Painted with single stage enamel. Same with my '67, which was painted 23 years ago. No issues at all. That said, today, it is industry standard to install the filler OVER the primer.