Epoxy thickener

[alex robinson]

I was wondering about ways to fill cracks on part turned bowls. Has anyone tried any of the various ways of thickening epoxy?

I would like to turn casting epoxy into something the consistency of big boy filler or polyfiller. Fortunately I would like it to be kept black, not clear!

A quick look up suggests 5 possible options:

1) Polyester fibres eg:https://www.neillsmaterials.co.uk/product/epoxy-thickener/

2) Colloidal silica
https://www.ebay.co.uk/itm/23539227...ar=535458902105&widget_ver=artemis&media=COPY

3) Thixotropic silica
Unsure how this differs from the previous option. Possibly the seller just chooses to highlight a different interesting physical property of the same product
https://www.easycomposites.co.uk/fumed-silica-thixotropic-powder

4) Microballoons
This sounds unpromising as it implies it won't leave a smooth surface.
https://www.ebay.co.uk/itm/30312814...gcasv-DSI6&var=&widget_ver=artemis&media=COPY

5) Wood dust / charcoal etc
Any random powder as a bulking agent.

 

[Orraloon]

Wood dust has been my go to and have seen some use coffee grounds and even brass filings. Small cracks I use CA glue with a bit of wood dust. Larger ones get 2 pac epoxy glue as that is already thicker than resin. I have not gone down the casting road filling large voids or river table type stuff. Sometimes you may have to build up the fill in layers if things are a bit runny.
Regards
John

 

[ChaiLatte]

I am having some difficulty seeing how a thick filler would achieve a good job.

By its nature, a crack is generally V-shaped, meaning its width decreases to zero at its base. A thickened filler will never reach that base and at some point will bridge over the crack only partially filling it and not locking the two sides of the crack together.

To me, the _least_ viscous form of filler would suit, so that it fills the crack like liquid in a container. Sure, you have to orient the part the correct way until the filler dries but that is not too difficult for a part-turned bowl.

That is the main benefit of a pourable epoxy - that it penetrates and impregnates while liquid but sets solid after a defined time. A thick filler is only useful on a vertical surface like a wall where it is challenging to make it horizontal without using a JCB.

If the nature of the crack is such that the filler will run out at one end as fast as you pour it in at the other, you dam or block off its exit (caulk, silicone, putty, plasticine, modelling clay, blu-tak, tape, etc.) before you start.

 

[kinverkid]

I'm assuming that the curve/shape of what you want to fill may make it difficult to dam as suggested by ChaiLatte and what I would also have suggested but I have been in situations where I have had to make a large dam and mix a larger amount of resin than required to fill the crack because of the curve of the surface. Filling the crack before turning is one solution but also wastes resin because you will be turning it away. I would suggest mixing your resin, pouring some into a separate cup and warming it up as well as the crack with a heat gun. This will make the resin 'runny' but will allow it to seep into the smaller voids within the crack. I'm now entering my theory because I've never used silica thickener. I usually use coffee granules. Add the silica (I took advice from my builder nephew here) to the remaining resin and paste it into the crack. My thoughts are that this will stop the resin from pouring down the side of the turning but I would imagine that bubbles may find it hard to rise (as it sometimes does with the coffee granules) to the surface so there may be a bit of 'topping up' filling if you come across some after resuming turning. This is where UV resin is your friend but use it clear, not black unless you buy specially coloured black UV resin.

 

[Keith 66]

Thickening epoxy is usually done with colloidal silica, Sold in horribly expensive cardboard tubs by epoxy companies like West or SP , The same stuff can be bought for a fraction of the price called Cab o sil, from fibreglass suppliers. adding silica to resin thickens it so it does not sag, but the resulting cured epoxy is hellish hard & hard to sand. Its also whitish in colour. You can get black pigment to add to the mix. Beware polyester & Epoxy pigments are not always the same.

 

[Jake]

West do some designed for fairing so more readily sand-able (410 from the MSDS is polymer rather than glass microballoons combined with unspecified colloidals). But my go to is always cotton microfibres, rather than just a filler they give gap filling strength to an adhesive bond. Sandable (by machine) and OK in with pigment but haven't tried to get black - as they are white you might not get past very dark grey. (But, thinking about it, you could probably pre-dye them black and dry them out again before use but that'd be 'long' as my son says).

 

[alex robinson]

I am having some difficulty seeing how a thick filler would achieve a good job.

By its nature, a crack is generally V-shaped, meaning its width decreases to zero at its base. A thickened filler will never reach that base and at some point will bridge over the crack only partially filling it and not locking the two sides of the crack together.

To me, the _least_ viscous form of filler would suit, so that it fills the crack like liquid in a container. Sure, you have to orient the part the correct way until the filler dries but that is not too difficult for a part-turned bowl.

That is the main benefit of a pourable epoxy - that it penetrates and impregnates while liquid but sets solid after a defined time. A thick filler is only useful on a vertical surface like a wall where it is challenging to make it horizontal without using a JCB.

If the nature of the crack is such that the filler will run out at one end as fast as you pour it in at the other, you dam or block off its exit (caulk, silicone, putty, plasticine, modelling clay, blu-tak, tape, etc.) before you start.

I agree that low viscosity is good where the cracks are narrow and it is important to stop the piece from splitting. It is still a pain using something runny when they are lots round the diameter of a piece - each needs to be filled and left to set before the item can be turned and the next one tackled, leading to a lot of waste. Also, if you are filling a curved surface, the caulk has to be built up to the level of the highest bit of the curve.

I am not particularly worried about the strength aspect - it is more the aesthetics. This is an irritating cherry root ball I am working on:

The grain is all over the place (and full of stones it has grown round so perhaps the most tedious / least fun thing I have ever turned) and I am not planning on making the walls very thin, so I don't worry about the cracks spreading. The wood is also soft in places, so I couldn't use a thin black epoxy first as it would mean too much staining (would need sealing with clear). Something that could be spread on and worked into the cracks as a paste would nicely highlight them.