Experience/Tips for large DPM module building?

Don’t use epoxy. It’s styrene. Use a styrene adhesive or MEK. It’s way faster and welds it together as one piece. I think the directions say to build horizontally and then add courses vertically. As long as you start with a flat bottom you can kinda go up in any manner you want. I tend to do the opposite and build vertically so I can get the columns in line. You gotta get the edge of the bricks in line vertically not the panel edges.  Once your first vertical is lined up, the rest of the wall should go fairly easily. Sometimes there is variation in the width of the gluing flange. I would build full walls before joining corners.
File the tops and bottoms so they’re flat and smooth. The sides, take the flash off of them. They don’t touch each other when you join them with the columns.  Always glue against a metal ruler or a level so you’re straight.
The columns can be longer than the floors. It is better to get the top of the column right and file the excess off the bottom. 
Have lots of angle blocks or 1 2 3 blocks on hand to keep everything square when you assemble it.  Long walls probably need bracing to keep them flat. 

Thanks Boilermaker, it seemed that lining up those columns was difficult when trying to build horizontally first so glad to hear you've built vertically first.  Good advice on the corners and other details as well...

I tried Plastruct plastic weld but it didn't seem to hold very well.

Dave:

It has been a while since I last built a DPM structure. I have attached a file that may help. I think I used Plastyruct Plastic Weld with no issues but can't be sure.
The most important thing I remember is having to remove the 'draft angles' from the sides of walls otherwise they would not fit properly. A sheet of sandpaper (80/100 grit), a good metal right angle like the 123 block to keep the wall square to the paper is all that is needed. A few swipes and you are good.

This is the first of 21 videos Woodland Scenics put out on DPM kit building - https://www.youtube.com/watch?v=1wd_ZI56xU4 Just beware with respect to glue the narrator is talking about HO kits which may be a different plastic then the O scale kits. If you don't want to go through the videos try the attached file.

Joe

Dave:

Just found this -

I purchased a DPM O scale building kit and one of the walls is warped. Is there any way to flatten it?

Older Korber kits, also need wall panel adjustment.  I used a heat gun, (high intensity hair dryer) As mentioned, heating the piece completely through is important.