AS LONG as the boards are in good condition, you should be able to use a wood scraper, sandpaper and a handheld electric sander to clean them off and make them smooth.
Remember to bang down any protruding nails with a nail punch and hammer – using a nail punch stops you damaging the boards with the hammer.
Boards in a worse state will need to be sanded with an industrial floor sander.
Hiring a sander – and an edger for where the sander can’t reach – isn’t particularly expensive, but it’s hot, hard and dusty work.
Afterwards, you may feel that as you’ve gone to all that effort, it’s a shame to cover the newly sanded boards with paint.