OK, what part of this process am I getting wrong (body work)??

[Kroggers]

I am trying to do some fixing and cleaning up on a fibreglass bumper that I picked up. Thinking this would be a simple job, I went to the local car DIY shop and purchased a tube of body filler (the stuff you get with a tube of hardener that you mix), I am happy with the mixing and spreading on the bumper. The problem comes when it is dry and I am sanding with wet & dry to get a smother and nice surface to paint the primer on. The section with the filler is full of lots of small holes, as if the filler hardens with lots of air bubbles

I have now spread filler and sanded the bloody bumper 4 times, and every time I am left with an area full of microscopic holes! I am getting close to mixing up a load of glue to spread over the area in an attempt to fill all the holes...

So, you body experts out there, what am I getting wrong here??

[Kroggers]

Hmmmm, think I just managed to answer my own question!! I need to use something referred to as "auto-body glazing putty" over the filler to give me the perfect surface

I am off to the shops to see if I can find some of this stuff....

[Thomas Owen]

Could the holes or bubbles be caused by moisture? What is the weather like where you are doing this work?
Good luck -

[stormmaster]

Number 1.....do NOT wet sand plastic filler.....it retains moisture and will shrink over time....Number 2....it sounds like you are mixing in too much hardner....and overmixing....adding too much air into the mix....do not "stir" to mix....fold....Number 3....it may be too hot....temperature can cause the filler to harded too quickly...causing air bubbles.....again....less hardner....#4.....what brand filler are you using....go to a paint supply store....get some good quality polyester filler....you will pay more.....but it is worth it.....you can also fill the holes with glazing putty....

There are a lot of factors.....a lot of trial and error....the good part....it sands off and can be redone....

[stormmaster]

Number 1.....do NOT wet sand plastic filler.....it retains moisture and will shrink over time....Number 2....it sounds like you are mixing in too much hardner....and overmixing....adding too much air into the mix....do not "stir" to mix....fold....Number 3....it may be too hot....temperature can cause the filler to harded too quickly...causing air bubbles.....again....less hardner....#4.....what brand filler are you using....go to a paint supply store....get some good quality polyester filler....you will pay more.....but it is worth it.....you can also fill the holes with glazing putty....

There are a lot of factors.....a lot of trial and error....the good part....it sands off and can be redone....

[stormmaster]

FYI.....filler should be sanded only to about 220 or 240 grit....then prime and block....reprime...and block some more....until you get to 400 or 500 grit paper....this will result in the best finish and will allow the primer to have a good tooth to bite into....sand it too smooth and the finish coat will not have anything to bite into....the finish coats can be wet sanded to 1000 and 1500 or finer.....but I would not recommend that unless the rest of your car has a show car finish.

[Kroggers]

Thank you for all the info Don. I have been to the shop and purchased another brand of filler, which is labelled as "fine" along with a product they do which is a filler in a spray can. According to the shop assistant these two products will give me a good finish.
Thank you for the advice about not wet sanding - that is what I have done

Can I also assume from what you have said that I should in the end use several layers of primer to fill any small holes etc by laying down several coats and wet sanding between each layer?

[stormmaster]

Dont wet sand primer either....there is a chance that you would get through to the filler...and allow moisture to get under primer....wet sanding should only be used on finish coats or clear if you are using 2 stage paint....a good high build primer (PPG k-36) is a good primer to fill sanding marks an small pinholes....I suggest skim coat of glazing filler to fill pinholes before priming....if you follow above recomendations you should not have too many pinholes....remember...dont use too much hardener....and keep water away...

[Kroggers]

Thank you for all the good advice and support Don. I did exactly as you said, to fix the parts requiring work. Unfortunately I left the other parts alone only to find that the primer reacted with whatever paint the PO had used. Now I have to strip it all off and start again

Another lesson for me - always remove or strip the old paint!!!

[Kroggers]

Thank you for all the advice so far Don. I have now managed to get all the filler done and block sanded. Next step is to lay down the glazing filler (cool stuff, comes in a spray ). Here are some pictures of progress so far.
Before block sanding:


After block sanding and ready for glazing filler:


I will post more progress pictures as I take them. Any additional advice you can give as I progress would be greatly appreciated