Bondo Advice

[MacMcMacmac]

I'm trying out my Bondo skills for the first time and it is a bit of a frustrating experience. I have found that you just don't fill the dip and be done with it, you have to actually cover a fairly broad area and blend it all in. Naturally, I find many dips and valleys once the sanding commences, which has led to at least 3 do overs. I think I am nearing the end (ha ha) but I am wondering what kind of finish the Bondo needs before I can paint? I know I cant have any abrupt edges between the Bondo and the surrounding metal, but the surface itself, does it need a 1000 grit polish before the primer? I'm resolved to being disappointed with the first coat, but if it comes down to having to strip and re-sand the entire tank, I will be pretty disappointed.

 

[WideAWAKE]

Maybe post some pics of what your tank at the moment.

But typically when I use bondo (I actually only use evercoat, as I find it flows better with less pits than bondo), it’s a fill coat (on big areas with a squeegee) and a rough sand back to say 220/320 ish depending on the circumstances. Then do a skim coat to get all the little pits/ small things missed.

Sometimes it’s 3 rough coats for things that are hard to get to and no clean way to apply it.

Skin coat can usually be knocked down and feathered in with 320-400 and primed (some primers may require more/less work to prep for).

You want it feathered with no edges.

Lots of good paint work done here so maybe others will give a run down on their process.

Tank after a few rough fills and sanding. Came from India (a lot of areas to fix)

 

[mike in idaho]

240 grit, then two light to medium coats of primer-surfacer. Give it time to fully dry(important), then go over it with 320 grit, if you see bare filler or metal, stop right now, and spray another coat of primer-surfacer. Repeat until the 240 grit scratches are gone and the surface is smooth and even. Avoid piling on extra thick layers of material that just slows the drying process, it shrinks a bit as it dries, and you don't want scratches showing up after your paint job gets a few weeks old and finishes drying. Never wet sand primer, it absorbs water and causes problems later. I never use paint stripper, nasty stuff, gets into the cracks and crevices and comes out to haunt you when the paint goes. Primer sticks to properly sanded existing paint layers(in decent condition, of course) better than bare steel anyway.

 

[jpdevol]

Naturally, I find many dips and valleys once the sanding commences

In addition to the tips above, I find to get a smooth/straight surface requires a sanding block of appropriate size and shape - can't do it with fingers only. Larger & longer blocks for larger areas.

And depending on the size of the repair, body putty faired as possible and then a glaze compound (a smoother filler for the filler) and more fairing (sanding)

 

[mike in idaho]

I use a long board, like this one: https://www.harborfreight.com/17-in...Fj9qIbQWYE0-DurgFXUrb9P2bvRVtEJRoCLIIQAvD_BwE makes it a whole lot easier blend the filler without leaving dips and valleys like you get with a short block.

 

[MacMcMacmac]

Here's the mess.

The darker stuff is contour putty. It's convenient but it seems to all disappear rather than fill in the little scratches. Speaking of which, you can see the swirls left by the electric sander. They are clearly visible through a trial shot of paint. I was thinking of taking the tank to work and bead blasting all of the non filled area to get to bare metal. If there is a spray on primer thar can fill these, I would rather go that route. It's the last thing to get done before Tetanus is road ready.

I am filling I the badge are and u am trying to figure out a good applique to stick to the finished tank, so any imperfecti9ns here are not important.

The side of the tank had a sharp shallow crease. I filled just that but when I sanded it had a halo of virgin paint all around it so I knew it was still low. I probably went overboard.

 

[WideAWAKE]

If that were mine and I was attempting to remedy the situation here is what I’d do.

Ditch the power sander.

Get a med/firm block like PJ posted and sand it with 220 all in the same direction/the natural flow of the tank and contours. Get it to a uniform looking finish.

You can dust it with paint/guide coat to get a good idea where you have highs and lows.

I’d then get a thin yellow squeegee (5-6” wide) and skim the whole area/areas that need to be address.

Then I’d come back with 320 on the block and sand the same way.

May knock it flat with med block and then switch to a softer one for the contours.

A filler primer will fill in those scratches.

Just an idea

 

[mrtwowheel]

RM trained autobody tech here, 45 years removed, but I still like to sculpt. My first coat, when thick is needed, is usually chopped fiberglass filler, shaped with a course sanding wheel. Then the first coat of plastic filler, sanded with 60 grit on a block. Following coats of plastic filler also sanded with the 60 grit. I wipe each sanded coat with a cloth while high pressure air blowing off the dust/pinholes. When my edges and contour are right I primer. I NEVER use any grit finer than 60 grit on the plastic filler. Three coats of WET sandable primer palm sanded, no fingers, with 220 grit. I avoid the putty, but if you use it apply it over UNSANDED primer. 2nd three coats of primer are palm sanded with 400 grit. If you can't get your filler to metal edges right.......then you are oversanding and/or using too fine of a grit.

I usually use a block on the first shaping coats, then I palm sand the last coats of filler. Paper tucked between the thumb and forefinger, paper locked in with the thumb, sanding with a side=to-side motion mostly. when sanding in a push/pull motion ease off on the finger pressure, fingers cause grooves.

 

[MacMcMacmac]

Thanks a lot. I knew this would be the most frustrating part of the job. I can swap and replace parts and splice in a new TCI till the cows come home but paint and bodywork are always a challenge.

 

[MacMcMacmac]

In the light of better info, sometimes, you just have to start again. A fully blasted tank is so much nicer to work with.