Prepping Holes in Really Soft Rock

I encountered rock today that was so soft that as I brushed it with the usual plastic brush, the hole continually grew in size and no amount of brushing reduced the amount of debris that would blow out with a pump blower each brush/blow cycle.  Usually to prep a hole w/AC100, I brush and blow 5 cycles to thoroughly remove dust prior to installing resin and the bolt.  With this rock type it seemed fruitless to try and do that. (though I did) 

The best method I've been able to brainstorm is to do the usual cycles and pump really thoroughly to remove as much dust/debris as possible at the end of the last brush/blow cycle.  Are there other best practices for rock this soft?

I'd go somewhere else

Wash it out with a squeezy bottle and tube then blow it dry(ish). Use a resin made for wet holes.

Alternatively after cleaning as best you can squirt some resin in and with a thin rod or similar smear the resin around the inside of the hole so it picks up the loose grains and they mix into the resin.

For glue ins in soft rock I do 3 or 4 brush/ blow cycles with a wire bore brush and hilti pump blower, and just really pumping the sh*t out of it on the last blow, until you don't see any powder blowing out at all (usually about 20-30 seconds of vigorous blowing).

For mechanical bolts I use a plastic brush instead of a wire brush so as not to enlarge the hole and only brush one cycle, but still blow it vigorously for 20-30 secs prior to install.  But if it's as soft as you're saying and you already use a plastic brush, glue in is probably the way to go

Edit to add make sure you're finding the densest section of rock prior to drilling by tapping with a hammer.  A couple of times I've hit a soft patch while drilling, but moved over 6 inches or so and drilled a new hole that was way denser.  Then just patched the bad hole with some of the extra glue

I have found canned air to work better in soft rock, then the hole is st least free of surface dust. I think that's the best you can do? Also spin the bolt a few extra times like Jim suggests.

Pick a different peice of choss to bolt. 

I just sent you a PM

Max Tepfer wrote:

I encountered rock today that was so soft that as I brushed it with the usual plastic brush, the hole continually grew in size and no amount of brushing reduced the amount of debris that would blow out with a pump blower each brush/blow cycle.  Usually to prep a hole w/AC100, I brush and blow 5 cycles to thoroughly remove dust prior to installing resin and the bolt.  With this rock type it seemed fruitless to try and do that. (though I did) 

The best method I've been able to brainstorm is to do the usual cycles and pump really thoroughly to remove as much dust/debris as possible at the end of the last brush/blow cycle.  Are there other best practices for rock this soft?

I've experienced this a few times. When this happens I avoid glue-in bolts. Instead I do minimal brushing and use the longest, fattest 5-piece bolts I can. This has been successful.

I've encountered a lot of rock like this and usually find the best method is to forgo the actual nylon brush and just use the blow tube itself to scrub the sides of the holes a bit between blows. This seems to remove most of the really loose debris without widening the hole much. I realize others might disagree about this being best practice though and the best actual method might be to use a "brush" with bristles that are less stiff or more of just a soft pipe cleaner or small towel end. The method Jim describes also sounds pretty good. 

Walk away from it, find better rock.