Bondo question. Does more hardener equal more hardness?


Well-Known Member
Like the title says, does more hardener equal more hardness, or just shorter cure times? Obviously if I don't use enough hardener The Bondo won't cure at all but will I have an easier time sanding if I use less hardener? Will I have a stronger product if I use more hardener?

Same question for rondo, if I use one hardener instead of the other will there be a difference in hardness? will it be harder if I use both?

I've been using bondo and rondo quite a bit lately and sometimes I could swear this stuff is impossible to sand, other times, not so bad. I'd like to know if it's just me.

also, has anyone tried mixing regular old wood filler and bondo resin?


Sr Member
It makes it set faster, but it doesn't make it harder.

And rondo gets harder if you use more fiberglass resin than bondo in the mixture (I usually do half and half).


Sr Member
It will accelerate cure time and will also change physical properties of the cured resin as a result of that faster (and hotter) curing. While the overall shore hardness won't change dramatically, what you will get is more brittle bondo.

If you're looking to make sanding easier, start by blocking the shape when the bondo is partially cured; it will hold its shape but you might be able to put a slight fingerprint mark in it. It will be less dense here and easier to sand roughly before finishing.

Wood filler is water based (generally) and won't mix or cure with polyester resins (bondo type fillers)


New Member
There is an aircraft grade Bondo that is lightweight due to the addition of glass microballoons. This is very easy to sand but more expensive. You can buy the microballoons and add them to any filler yourself, but be sure to wear a mask as you stir them in to avoid inhalation. They can also be added to resin to make a thicker, but more brittle, resin that is easier to sand.

If you need the opposite, chip amd crack proof, that is harder to sand as a result, get the filler with glass fiber added. I use this on corners and edges of a prop that is going to take abuse.

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