Anyone know if new resin will bond with cured resin?

As far as I know only CA glue sticks to resin. You can mix CA glue and baby power or ever resin dust with CA and fill things. However, resin dust is a seriously nasty thing if you inhale even small amounts. It WILL wreck you lungs. I won't play with the stuff. I only use N95 respirator when working with resins due the health issues associated with it. But, CA will bond resin to resin.

If you are using a liquid resin then I would assume it will bond, but how well.

BK

do you mean epoxy resin? like 5 min, 10min 2 part liquid?

Your question piqued my interest.

I think the problem with using more liquid resin to bond to resin is controling where it goes, unless you have the original moulds?

OLD school casting would have you chip used overpours, filling trees, failed casts, etc., to add bulk to larger volume pours, to economise on expensive resin, so i presume like with like will bond.
Don't use resin dust, not worth the effort, or risk. As I only wet-sand, then wash parts, to prevent breathing in dust.

For GLUING Resin to plastic or resin parts use 2-part epoxy glue or CA/Superglue.

For FILLING resin parts, CA, or talc, then drip resin on it, or 2 part epoxy fillers such as Milliput, or 3M Bondo/Holts Knifing putty (UK) or Deluxe Materiels Perfect Plastic Putty.

Jon_a_its

...OLD school casting would have you chip used overpours, filling trees, failed casts, etc., to add bulk to larger volume pours, to economise on expensive resin, so i presume like with like will bond...

I don't think using pieces of cured resin as filler in a new casting is the same thing as using a fresh batch of resin to try to fill pinholes, seams, or gaps on a piece of cured resin, though.  This cured pieces added to a mold to reduce the amount of volume you need to fill with the new batch of resin might react with the new resin, but for all intents and purposes, they may as well be inert, unreactive.

The Romans used to use a similar technique, by the way, when mixing and pouring concrete.  They would toss in small bits of pottery, including small pottery jars which trapped air, to save on the amount of concrete poured, without losing overall strength on the final cured piece.

But yeah, for filling those pinholes, seams, or gaps, I use Mr Surfacer, or 2-part epoxy putty, like Milliput or Apoxie Sculpt.  I like to use 2-epoxy cement for assembly, especially on things like attaching limbs on larger-scale figures.  I pin the joins, too, as necessary, for additional strength.

I am assuming you are using epoxy resin on a urethane casting.  Many casting resins, especially epoxy, but also urethane, leave a waxy residue.  Solvents like lacquer thinner, light oil, or high concentration isopropyl alcohol are more effective that soaps or detergents on that film.  The exception is Purple Power, a very powerful base (watch your hands with that stuff).