I have a complex protein (purified over gel filtration) with the GST tag.

I have other small proteins (Darpins) with Flag tag

I want to a pull down using the GST tag and check if these Darpins can bind the complex.

I am using the Chromotek GST agarose beads, and they show a lot of non-specific binding (binding to the Darpins).

I have tried using 5% BSA pre-blocking of the beads for 1 hour but it does not improve the condition a lot, it does a bit.

What other ways can I use to pre-block my beads?