How To Use 2 Part Fillers?

Whether you’re carrying out repairs to woodwork, metal, plastics, walls, even concrete, there are few better fillers available than 2-part epoxy resins. They’re incredibly versatile and have many advantages over traditional fillers. In this video I’m explaining how to use them and listing a few pros and cons against traditional ready mixed fillers.

Watch the full video here:

How To Use 2-PART WOOD FILLERS – GHTL#13 [92]

Watch this video on YouTube.

2 part fillers are an alternative to ready mixed fillers and offer many advantages. Common names for these types of filler include:

  • 2 part filler
  • Epoxy fillers
  • Epoxy resins
  • Polyester resins

These fillers set through an exothermic catalytic reaction between the filler and the hardener. You’ll feel it actually get warm as the reaction takes place and once it cools down again this is a good indication that the filler has set. Common brands in the UK are:

In America ‘Bondo’ by 3M is a common brand but please correct or add additional brands in the comments of the video above! I mainly use Bonda Decor Fill in the UK.

Advantages of 2 part fillers:

  • Better finish than ready mixed fillers
  • Easy to sand
  • Sticks to almost any substrate and can be used on wood, metals, plastics and even concrete!
  • Sets very quickly so can be sanded and painted in under 20 minutes
  • Can be sanded to zero
  • Can be painted with almost any paint
  • Takes stain pretty well
  • Quite resistant to cracking

Disadvantages of 2 part fillers:

  • Limited working time, as little as 2-3 minutes in some climates
  • Difficult to tool, try not to get it on your fingers!
  • Bonda is grey and takes a few coats of paint to turn it white
  • Quite expensive, 7kg tin is around £40
  • Has a strong smell – love it or hate it! The smell dissipates as the filler dries

I generally buy Bonda Decor Fill in 7kg containers and decant it into a glass jar. You’re best using glass jars with a metal lid – avoid plastic storage containers as the fumes from the filler can cause the plastic to melt over time.

I use 2 filler knives – a 1.5″ knife and 3″ knife. I keep these VERY sharp as it gives a better final finish on the filler. I clean & sharpen them using my belt sander however you can just use a piece of sandpaper. I just use scraps of laminate flooring as a mixing palette.

Mix the filler in the ratio of a GOLFBALL to a PEA. So a golfball sized glob of filler should be used with a pea sized squirt of hardener. Obviously you don’t have to use that much but if you keep the ratios the same that should work. The ratios aren’t that critical. Mix the filler for 20-30 seconds before use. As this is a catalytic reaction between the filler and hardener the mixing isn’t too critical as long as you’re not left with visible unmixed hardener you should be fine. If it’s very cold or the filler hasn’t been mixed so well it will take longer to set.

PRO TIP:  If you have filler on your knife never put this back in the filler jar if it’s already been mixed with hardener! Always use a clean knife or scrap of wood to get more filler out.

The hardener on your knife can trigger your full jar of filler to harden, sometimes VERY slowly but your filler will eventually be ruined. If you’ve ever come back to your filler jar / tin and found the whole lot has gone hard, this is probably why it’s happened.

Only mix as much filler as you can use – it sets very quickly and it’s expensive. Make sure whatever you’re filling is clean, dry and dust free. I find it easiest to apply the filler using my 1.5″ knife but then smooth out larger areas or match in to profiles using the 3″ knife. When filling holes drag the filler from the middle of the hole to the edge and and try to leave it slightly proud so it can be sanded down to match the level of whatever you’re filling. If you drag the filler right across a hole you’ll find it will ‘drag’ itself out of one side and leave a low spot – working from the middle out avoids this. If you do find any low spots or imperfections afterwards you can always go back over it with another very thin layer of filler. For very deep holes fill in multiple layers.

As soon as you feel the filler starting to harden stop applying the filler or you’ll wreck your finish. Clean your tools and mixing palette while it’s in a ‘gel’ like state, before the filler has hardened completely. Use one knife to clean the other knife when you’re done.

You’ll feel the filler go warm as the reaction takes place – once it’s cooled down again it’s probably ready for sanding. Double check by gently digging your finger nail in to the filler area (or even on some scrap filler from the same batch). If you can dig your nail in to it wait a bit longer before sanding. If you find it hard to sand the filler to zero this is often since it hasn’t properly set yet.

Once you’ve sanded it, prime it with an appropriate undercoat (I normally use Leyland Acrylic Primer Undercoat on woodwork) and then use your top coats as normal.

ALWAYS REFER TO THE MANUFACTURER’S INSTRUCTIONS

Andy Mac

Andy runs a busy little bespoke joinery business making everything from custom made furniture through to commercial display systems. He's been self employed most of his life, runs various YouTube channels and is co-host of the UK's first commercial joinery podcast.
Andy Mac