I am running a bit low on budget and mdf seems quite pricey locally for 8' planks.
the local store has spf 1x6x8 grade 1&2 for a good price but I'm wondering if I can use this for baseboard or if it will be a bit rough? I usually use pine and seal the knots then prime...

 

How thick is your casing? Most don't care for the base to be thicker than the casing.
Most of my house is trimmed with sawmill lumber [planed and routed] IMO the heftier base looks fine as long as it works with your casings.

 

SPF doesn't tell us squat. It's about as generic as laying "lumber". As for using it... that's up to you. What look do you want? What is the quality of the wood? Yes, I know you said #1 & #2 but have you looked at the wood to see it's condition? What tools do you have to clean it up and make it look nice? If whatever you are considering is rough you can sand it to make it smooth but you have to offset the labor & tools against purchasing a higher grade material that doesn't need cleaning up. I have seen some absolutely gorgeous pieces of lumber that could make quite nice looking moldings but that's a rarity but you might be able to pick through the pile and get enough good stuff for what you need.

 

I don't quite understand pilot danes comment that #1 and #2 spf 1x4 doesn't tell us squat... he seems to be picturing it perfectly. It's just like the pile of construction lumber that most any lumberyard would have. The boards will need to be picked through, just like he said. They will usually have planer marks in them, and small to half-dollar sized knots. They will require lots of sanding. And after you sand, and prime with latex primer, the grain will usually swell, so you will not end up with a nice smooth paint job like you would with #1 fir or mdf. Knots will also bleed and yellow over time, so a stain blocking oil primer would be better than latex.

Sure, you can use it, but yes, it will be more than a bit rough. If you want to do it on the cheap, you'd be better off buying a 4x8 sheet of mdf and rip it up on a table saw. And even if you did that you would be spending a lot of time ripping on the table saw, sanding your saw cuts and routering roundovers.

Not buying quality trim is a tradeoff of quality for labor, and in the end, won't look as nice.

 

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SPF is an acronym for spruce, pine, and fir. SPF lumber typically refers to dimensional lumber or engineered wood derived from coniferous trees in North America. While most SPF lumber comes from Canada, SPF lumber can also come from trees in the northern United States.Apr 13, 2020