I use BLO, turp., bees wax, equal parts. Once the wax is melted ( use a double boiler )and the ingredients are thoroughly blended, add a heaping tablespoon full of either concrete tinting powder, or chalk line chalk, in the color you prefer. The powder helps seal the pores of the fabric when it's all dry. If you like living I'd forget I ever heard the part about putting it in the microwave. Go to Harbour Freight, and buy a cheap heat gun, and hang the treated bag up in the shade, and play the heat gun over it until it is dry to the touch.
Oh by the way Harbour Freight has canvas painters drop cloths that will make a pretty good bag for cheap.
Hungry Horse