Glue-Ups...a few questions

There is a great spot for a glue-up near my place and I have already started putting it together but have run into some problems. Hopefully somebody can help me out.
First of all, the wall upon which the glue-up is going is a concrete wall. I had put several dozen holds onto the wall using Liquid Nails (Heavy Duty) with some success. After applying the holds and waiting one week, the hand holds held (mostly) well. Although a few came off under almost no pressure, most were able to support my full weight. However, every single foot hold came off. Kind of frustrating. So here are my main questions:

-Is there a reliable way to prep the concrete to make the surface more receptive (I already scrub the spot where I am about to apply a hold with a dry nylon bristle brush)
-Can someone suggest something better than Liquid Nails (ideally someone who has used Liquid Nails and can compare with another substance).
-Is there a certain technique I can use when actually applying the adhesive that might help

Thanks everybody, I appreciate your positive responses.
-AB

i have done the same thing to a wall near my home, i used a high strength concrete specific adhesive, liquid nails just wasnt quite strong enough, and just clean it like you already have, the holds i put on my wall were ready to climb in 6 hours, also note that mine were also put on a significant overhang (30-40 degrees) with no problems

Right on thanks Brennen. Can you tell me specifically what kind of adhesive you used?

PC-7 epoxy is what you want. There are some other two part epoxies you can use but you just have to make sure the epoxy will be thick enough after mixing that the hold stays in place while it cures. PC-7 is like tar after mixing and takes about a day to cure.

If PC-7 doesn't work, your concrete is the issue. You might try using a wire brush instead of nylon.

Edit: when I did it, I looked into high strength concrete adhesive and I couldn't find anything as cheap as PC-7 epoxy. It's around 11$ for a 1/2 lb can at Depot. Not sure how many holds you are putting up but 1/2 lb can do quite a few.

Thanks Greg, that sounds great. The spot I've chosen could handle a ton of holds, but my motivation for putting together a glue-up at all is primarily relating to training. I can't afford a gym membership, and sometimes two or three days go by when I can't find anyone to rope up with. Hence the glue-up idea.
Bottom line: cost is an issue but it sounds like the PC-7 will be doable for me as I'm not trying to make the world's most insane glue-up, just a solid place for me to train when I cant find a partner.

I understand that man. You might check out what other two part epoxies your local hardware stores have and ask someone how thick they are after mixing. There might be some cheaper options than PC-7. Like I said, just make sure the epoxy will be thick enough to keep the holds up while curing. PC-7 is the only one I've used.

You should check our RedHead A7 epoxy - two part, dries in minutes.

One trick is to "score" the backside of footchips to increase surface area. Check out Ian Powell's interview on ClimbTalk for more ideas.

Part of me misses the glue-ups of the Central Coast. Thanks for contributing to the community.

Bondo is all I have ever used and it works great.
Wire brush the wall
Score the back of the stone with a grinder in a tic tac toe pattern
Put bondo on the back of your rock and duct tape it to wall for a couple of days
Bring some rags to clean the excess bondo that ozzes between the rock and the wall