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.

 

[alex robinson]

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.

UV could be an interesting option to try. Also like the idea of the 2 stage fill.

It can be frustrating working with timber that doesn't come in neat flat plank form. You want to dry it after the initial turning as it would take forever to dry otherwise, but that means a much harder shape to work with!

 

[alex robinson]

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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).

Compared with doing each crack individually it seems wonderfully "short". I will give it a go. I have a huge piece of eucalyptus that is going to honeycomb spectacularly, and I have been putting off turning it until I have at least a possible answer to try!

 

[Jake]

The advantage of strength is that it will help the filler not fall out - the fibres will bridge between the edge bonds.

 

[NickDReed]

I've had some success with coffee grounds, but they need to be very finely ground. The issue as some have mentioned above is whether you can get it to down to the bottom of the crack, or whether you need to.

Milliput is something else I have seen that looks good, but I have no experience using. Again, whether you need it to penetrate to the bottom of the crack is key.

 

[treeturner123]

Car Repair epoxy, coloured black with Indian ink or paint. Works a treat.

 

[alex robinson]

Car Repair epoxy, coloured black with Indian ink or paint. Works a treat.

The answer might be simpler than I thought! How does it polish up? I had thought that stuf flike that didn't really take a shine?

 

[kinverkid]

Car Repair epoxy, coloured black with Indian ink or paint.

I didn't think of that one. Good call.

 

[recipio]

Have you tried black shellac sticks. ? They seem to run into cracks with a kind of osmotic attraction. i bought a box of them but I think it was from Lee Valley in Canada.

 

[Stigmorgan]

Partially tape over the crack with duct tape leaving the "highest" point open, pour in your resin or CA until full then complete the seal with more duct tape and move on the next, once done wrap the entire thing tightly in clingfilm/shrink wrap and go get a cuppa and wait for it to dry/ set.

 

[Ollie78]

What about a hot melt knot filling gun. I find it pretty handy and you can get many different colours of the sticks.

 

[RonnieI]

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:
View attachment 189497
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.

I think if you're leaving the walls reasonably thick I'd just turn and finish that as is, the cracks are part of its charm. If you're really worried about it exploding I'd run thin CA into the cracks (but don't fill them fully) and a quick spray of accelerator. That'll be enough to stabilise the cracks and stop them propagating while you finish turning.

 

[Bob Chapman]

Old laser printer toner makes a good black filler.

 

[Owd Jockey]

Hi Alex, I have tried a number of different methods to fill cracks/voids on my turned pieces over the years, including CA glue, PVA Glue, Milliput, epoxy resin resins etc, each with a measure of success. I have recently changed to simple wood filler, which I sometimes colour. The problem with 'pourable' resins is you are reliant on gravity doing its bit,however, like others have mentioned curvature of the 'piece' may require damming and or gentle heat to release air bubbles (Ok you could use a Pressure Pot to do the same thing).

With the wood filler method, I use Ronseal Hi-Perf Filler, you can work the paste into the voids, BUT you have to be quick as especially in summer it goes off quick, so little and often.