Basically, I want to avoid situations I've encountered in the past where the dyed epoxy seeps into the wood surrounding the void and creates a stain or blotch that is difficult to impossible to sand out. I've experienced this with boards that exhibit spongy or lightly-rotted areas around the void I'm filling.
The part I've bolded is the main issue here, other than that staining of surrounding wood shouldn't be a big problem.
If you can avoid using wood with punky areas that'll help a lot! Although obviously this isn't always possible and there are some spectacular boards which have some punky wood present that it would be a shame not to use.
Are there any tricks to pouring dyed epoxy into voids without it creating blotches or stains?
There's no guarantee these will prevent the problem 100% every time but some of the following should be of help:
- Apply clear epoxy to the problem areas before using the coloured epoxy. The tinted epoxy goes on after the clear epoxy has set but before it has fully cured so it doesn't add a huge delay.
- Brush on a coat of shellac as above. This has the advantage that you can proceed almost immediately because shellac dries so quickly.
- Lightly finish the surface of the surrounding area to help prevent staining from the top down.
- Use pigments instead of dyes* to colour your epoxy (either in dry form or by using paints, which are coloured with pigments).
I would switch to tinting with pigment or paint regardless of whether any of the top three things is also done (although personally I would do both if erring on the side of caution). Another good reason to prefer using pigments is that they are much more resistant to fading than dyes.
Truthfully, I've had better luck using sawdust as a colorant
Nothing wrong with that IMO if the effect is deliberate and what you're after.
Just to mention you can do both, tint and fill the epoxy. I do this frequently to tweak the colour of wood-dust fills since I don't have a large enough stock of fine sanding dusts to suit every requirement. Most frequently I'll do this to make a mix darker but sometimes need to shift the mix slightly towards yellow or orange/red as suits the wood in question. Very occasionally I'll add a dab of white to lighten a mix slightly.
I was asked to add a bit on the following from a Comment on a duplicate Question.
Note on light stability
Most dyes aren't lightfast, the majority will fade or alter colouring in some other way (shifting hue or going duller) to some degree with light exposure. This is an everyday issue with dyes used to stain wood because so little dye is involved and it sits in a shallow layer of the surface fibres of the wood.
When used to colour an epoxy fill however this same degree of sensitivity to light may or may not be something to worry about. With deeper fills on interior projects the total amount of dye present and the depth to which it extends mean there can be minimal colour change over time, especially since interiors are generally a low-UV environment. But if used for a shallow fill, even more so for something of thin section (light can get to it from two sides), and especially for anything that will be positioned near a window this is something to bear in mind. In any of these situations you may want to go a different way to help ensure colour stability.
Pigments on the other hand are now generally light stable. While you usually won't be able to find out what pigment or pigments were used in a paint this will be particularly the case in those intended for exterior use. When buying pigments in dry form there should be some sort of lightfastness rating so here you can select for this property. In addition to being more lightfast themselves pigments provide a secondary benefit as they help protect the surrounding epoxy from the UV by absorbing it.
*You refer to dyed epoxy throughout and I'm presuming you're using the term literally. If you switch to using pigments some of this problem will go away because although pigments are small particles, some very tiny, dye is on a totally different scale — individual molecules. This is why we get much better penetration from wood dyes than many stains, which are pigment-based (although some stains use a mixture).