I found that you get better salvage from doing the demolitions and car transport guerrilla actions than you do from straight up just demolishing EDF buildings. Plus, if you beat the 'pro' times on either of those, you'll get double the scrap (if you've already beat them at regular speed and beat them in Pro after that you'll get the scrap reward a second time). I'd also suggest looking for the ore deposits, if you buy the upgrade you'll get 12 scrap for each one you destroy.
Also, if you raise the morale of the town up a lot before doing the Red Faction missions, you'll get a lot more bonus scrap for completing them. I've also made it through much of the game (trying to finish freeing Eos now) without using the arc welder, grinder, or proximity mines. I find myself using rocket launcher, shotgun (peacekeeper or the marauder one) and remote charges for a while. At this point it's rocket launcher, nano forge and thermobaric rockets or remote charges.
I don't really love the game but since I made it to Eos, I've been finding myself enjoying it a lot more. If you were in a slightly denser city, the destruction would be amazing ... as it is, things are just a bit too spartan. Still, it's a great rental and would recommend it in that regard.