Please help
I am working on some badly cast resin parts full of holes. They are too comlex for me to scratchbuild them. The more I sand and fill them the more holes appear. What kind of putty shold I use. Are there any special tricks for this situation? Thanks in advance

Are the holes really Small??? If so a Spray with Mr Surfacer might help. or you could try some "Sandable Automotive Spray Primer"

HTH

Thank for the response. The problem is that the part has many hard edges. That large bubbles appear near the edges. I have tried to eradicate then with Milliput. While doing the sanding I exposed many smaller bubbles on the flat places- in spite of being extremely careful- and, naturally, new large bubbles along the edges. I did the refilling-sanding at least tree times but still not perfect. I am pretty sure that others have already had this kind of problem with resin. What would be the most reasonable approach? I have tought of the surfacer too, but it is not enough for the large bubbles.

Instead of sanding back all the milliput and risk further damage (or opening small pores), use a water-dampened cotton bud/top to wipe away the excess millput and smooth it!
For small holes use Mr. Surfacer 500 in these areas ... then useing cotton tops/buds wipe away the excess, but using acetone-free nail polish remover instead of water.
No sanding at all, only filling the holes. You may have to repeat several times as both tend to shrink with this method, but at least your not opening more bubbles or damaging already bad resin!

The acetone-free nail polish remover, also works with tamiya and humbrol puttys.

You can also try packing the holes with baking soda, then touching a drop of CA to it. It will instantly fill the hole, and if you sand it immediately, the CA is much easier to sand than if you wait. This works well on corners, whee you can pack the baking soda into the right shape.
After a few AEF Designs kits, I considered myself an SME right away.

Thank you for your responses, youve really helped me out.

There is also the option of using .005" (.13mm) thick styrene sheet and gluing it to the surfaces with the problems. Possibly more work, but frequently not, especially if the surfaces don't have a huge amount of added detail. Filling, drying, sanding can take a lot of cycles if you have to work with a crappy casting. You can easily lose days and change the shape of the surfaces in question with repeated sanding. Sometimes it's easier to just lay a sheet of styrene over it and call it a day.

Frequently, resin surfaces with that many small pin holes near the surface are tough to sand after filling without, as you say, opening up new holes. It becomes a never ending search and destroy mission. Plating over with styrene puts all that to rest.

Paul