Does Body Filler Absorb Water?

There’s nothing worse than seeing rust start to bubble through your paint after you’ve repaired a panel. It’s a sure sign that moisture has gotten into work zone. And that begs the question – does body filler absorb moisture?

To help answer that question, Eastwood’s senior product manager Mark tested two types of body filler for water absorption. He did this by curing a small batch each of Evercoat’s premium polyester filler and their Everglass filler with fiberglass strands. Once the two blobs cured, he weighed them, trimming the heavier piece to match the weight of the lighter one. The two pieces were then soaked in water for 24 hours and reweighed to determine if they had absorbed any of the moisture.

The standard polyester filler did, indeed, absorb a small but measurable amount of water in that time. The fiberglass fill, however, measured out exactly the same as it started. Both fillers use a catalyzed polyester resin for the base, so why the difference? As Mark explained, “The glass strands [in the Everglass] eliminate micro pinholes that would have allowed moisture to be absorbed.”

He further explained that the glass strand filler is not intended as a “finish” layer of filler because it’s exceptionally hard to sand. However, because of its ability to prevent moisture intrusion, he recommends using it primarily over weld beads. It’s especially useful on patch panels and areas where you may not be able to work the back side of the weld. The glass strand filler should be used as a skim coat to seal the weld joint and topped with traditional polyester filler to form a finish-worth surface.

Preventing Corrosion Under Filler

Depending on the nature of your repair, you may not need to use a glass strand filler at all. If you can access both sides of a weld, you can properly grind, fill, fill and prime your seam. This will help prevent moisture getting to your filler in the first place. Here are some additional tips to help keep moisture from ruining your bodywork repairs.

Seal with epoxy primer – Apply a high-quality automotive primer over the cured body filler. A catalyzed (2K) epoxy will bond best to bare metal and will feather into existing paint. Primer acts as a barrier, sealing the metal completely and providing the necessary adhesion for filler coats.

Proper surface preparation – Ensure the area being repaired is thoroughly cleaned and dried before applying the body filler. Any moisture or contaminants on the surface can lead to adhesion issues.

Applying filler correctly – Follow the manufacturer’s instructions for mixing both filler and hardener in the correct ratio as well as applying it to the body. Make sure the filler is spread evenly and smoothly over the surface without leaving any air pockets or gaps where moisture can collect.

Complete curing – Allow the body filler to fully cure according to the manufacturer’s recommended time frame, factoring for temperature and humidity. Proper curing ensures that the filler is hardened and less susceptible to moisture infiltration.

Quality paint application – Apply paint in multiple thin layers rather than one thick layer. Ensure each layer is fully dried before applying the next one. Properly applied paint forms a protective barrier that helps seal the body filler from moisture. Whenever possible and appropriate, paint both sides of the work to ensure the best protection.