I have tried VF heaters before and after i insulated my garage. Before insulation it wasted fuel and didn't ad any comfort. After insulating it caused serious moisture issues within hours as well as CO. Running with the doors and windows open lets most of the heat out. With what you waste in fuel you could pay for a vented heater.
I have a 45k Mr Heater in a 24x24 insulted building. I leave it set to 45deg or so when I'm not out there (it runs about 6 minutes per day to maintain this). If It's real cold out and I'm working out there it maintains 65 by running a few minutes an hour. (CHEAP)
2 interesting observations.
1) If I'm using solvents or running a vehicle I partially open a door and window to ventilate the work area. I also have opened the attic door at times for 15 minutes or so to carry parts up and down. When I do either my heater will run continuously. and the garage will cool right down.
2) I run my generator about once a month for maintenance purposes. Generator is running outside, cord run inside with a 1500w electric heater plugged in for a load, (Heater inside garage). The garage will actually warm up from 45 to 50-55 in less than an hour.
VF heaters are a poor option that cost more in fuel, provide CO and moisture and add very little comfort if you need to keep a door open to run them.
For occasional use in an insulated space, I would go electric. low cost to buy, no heat loss with doors open etc. I pay the ridiculous rate of .16 cents per Kwhr. Running one 1500 watt heater would cost .24 cents per hour. if you have 2 15 amp circuits you could run 2 to heat it up quicker for .48 cents per hour. Hopefully your rate per Kwhr is lower up there. 1 heater for 20 hours over a weekend is $2.40. Or get crazy and say 2 heaters for 20 hours $4.80 for a weekend. (your Rates per Kwhr may be lower than mine)
Now let's look at a 30k btu propane heater. 1 gal of propane contains 91600 btu, at say $3.50 per gal. Vent free heater is 100% efficient so it will cost about $1.15 per hour or $23 for the same 20 hours, to breath CO, work with the doors open, and soak your tools and insulation with moisture.
Your garage is attached? do you have hydronic heat in your house. You could run a loop out to the garage and install a heat exchanger with fan.