Polyester Body Filler is one of the most important products used in our shops but sometimes improper use can result in failures and callbacks.

When adhesion problems occur soon after a repair is performed is normally means two things have gone wrong.

Either there was too much or too little hardener used during the mixing of the filler.

Or there was improper preparation of the substrate.

Although polyester fillers can be used on a variety of surfaces with good results you should never use them on galvanized parts. Galvanized parts should either be coated with primer or for best results you should grind the galvanized surface until you reach bare metal. Working with galvanized metal requires special techniques that we are not going to cover here.

Another reason your filler may fail is the temperature of the substrate that you are applying it to. Most often the problem occurs on hot surfaces but it can also happen below 60 f.

When working on a vehicle you should always do so in the shade or in a shop. If the surface feels either hot or cold to the touch then you need to improve the situation before you apply any filler or paint.

The final reason polyester filler can fail is when you attempt to force dry it too quick. I personally never force dry filler but there are sprayable ultra high build primers that are basically a polyester filler type product. I would still give these products their full out gas / flash times and not force them dry with heat.

Moisture and rust behind the repair can also cause failure of polyester filler however that is not quite the same as pure adhesion failure.

How do you prevent adhesion failure in Body Filler?

You should always follow proper mixing directions.

Never use cardboard for mixing.

Never mix the product on the vehicle’s surface.

Never apply filler to a surface that feels hot or cold to the touch… that doesn’t mean so hot you can’t touch the car it means over 95f or warmer.

Always prepare the surface with a proper grit sandpaper.

Clean the surface of dust and oils before applying filler.

Use a skim coat first if you need to add a layer thicker then 1/8th of an inch.

Never apply filler thicker then about 1/4 of an inch.

Use fiberglass enhanced products such as Tiger Hair or Kitty Hair for thicker applications and standard light weight filler for a final skim coat.

How do you cure repairs with body filler adhesion problems?

If a portion of the repair that you previously made fails and you can see by sanding / grinding that the surrounding area was applied at the same time then it is best to remove all of the filler back to metal and start the repair again.

Remove all filler from the previous application and start over.