Almost every place listed under "Janitorial Equipment & Supply" in your yellow pages phone book will sell you a high productivity pad for between $10 and $20. Along with the pad, buy a "sanding disk":

http://www.contractorsstore.com/site...reens-20_1.jpg

for between $5 and $15. Sanding disks come in different grits, from about 50 to 400 grit, and they're made of the same material that drywall sanding screens are. Get the coarsest grit screens they have. The pins on the bottom of the drive block turn the high productivity pad, and the stiff nylon bristles of the high productivity pad turn the sanding disk.

And, finally, the rental machine should also come with a "skirt" which goes on first, before the drive block, high productivity pad or sanding disk. It covers all the rotating stuff so that if you bump into a wall or something, you don't leave a mark.

Now, floor machines come in different sizes. If the rental machine is an 18 inch machine, then you need to buy an 18 inch high productivity pad and an 18 inch sanding screen. Those numbers refer to the diameters of the drive block, pad and screen.

Use that to sand the paint off your porch, but be careful to check that the pins on the drive block aren't penetrating through the pad and screen so that they're being worn off by abrasion with your porch.

People use this arrangement all the time to sand the poly off hardwood floors before switching to a belt sander to sand the wood in the same direction as the wood grain.

Remember to check the pins on the bottom of the drive block often to ensure they're not being worn off. I ruined a drive block on my sister's floor by not checking often enough.