What resin should I use that can easily enter small canals?

Hello 😊. I have a cast of a tree that I’d really love to fill with red resin. The thing is that whatever material I use has to easily go through several pipes that branch off from one another, and progressively get smaller. The main one (the trunk) has a diameter just large enough to put your thumb in, its main branches are approx. the diameter of a pencil, their branches the diameter of a toothpick, with those at the tree’s termination about the diameter of a sewing needle 😩.

This would be my first time working with resin so I have no clue, but it seems to me that I need resin that’s kinda watery and sets to very hard, but I couldn’t find anything on the internet. Could you, in your excellent knowlege, suggest a resin / technique that could work for me please 😅 ? Thank you 😊