Best way to flatten joined boards

The easiest way to flatten them is start with flat and square boards. flatten one face on a jointer. Then with that face on the fence, joint the edges. Run it through the planer with the flat face down and it will come out nice a flat.
You can joint them by hand as well. When you lay the boards back to back and plane an edge, the edges fold back together to make a perfect pair. Go from to /\ to ||. Picture them laying side by side and lifting them up in the middle of the joint so the outer edges end up on the bottom and the two backs come together, like a folding door.

Take some time to learn to make them flat before gluing them up. It will help quite a bit.

Already glued them up and they need to be flattened? Lay it in a manner that helps keep it stable, such as crown up. Plane across the grain to level it out. Use winding sticks to help flatten it end to end. use a straight edge (level or framing square) to help reveal the high spots. Mark the high spots and plane them down.
A similar process with a portable belt sander works great too, though the sander can work a little too fast.
Take some time to learn to make them flat before gluing them up. It will help quite a bit.

You don’t need expensive tools to do this. I have a very nice, very flat cherry table that I made flat with a $12, small, Stanley block plane. A sharp blade and patience are key. Some people flatten panels with nothing more than a card scraper.

Sounds like the boards are already glued up and BBrown626 has given some good advise. I have a few old planes as well as some LN new ones. I like you did not know how to tune a plane, but I found that if you learn to sharpen the blade so it will shave hair off your arm it work good enough to flatten your boards. Once you can get 2-3 thousands shavings you will be on your way. It should not take to much planing if your as close to flat as you indicate. Planing across the grain is important to quickly get them even,then smooth them with the grain.You will find that working with a sharp plane is a joy.

for me? #5, #7, #5 1/2.

If that’s not an option, then what BBrown626 is describing seems like it should work rather well.

-- Mos - Twin Cities, MN -- Stanley #45 Evangelist - www.youtube.com/MosquitoMods

I use bench planes. They are the right tool for
the job. Trying to sand boards flat with a handheld
sander is possible for some belt sander experts,
but I don’t recommend trying it unless you can
accept a finished project that will always catch
the light a little weird.

You can make a tool called a sanding board, which
is a flat piece of MDF of other man-made board
with some sections of sanding belt glued to it.
You move the sanding board over the work
and it removes high spots. It is tedious to
flatten boards and doors this way but it
requires little skill.

Could you do something like project below?

http://lumberjocks.com/projects/56756

Hotbyte, That jig looks pretty cool.

I may not have been clear on the original post. The boards are fairly flat, I need to dress up the joint where the boards meet. I think I was a little sloppy when I cut the biscuit slots. There is a slight lip in the center of the joint, maybe 1/32” high on one side. The ends of the joint lined up flush and will only need light sanding. I learn something on every project.

Thank you everyone for the info. I used a scrapper for the first time and I was surprised by the amount of material I could remove.

I am dressing up my planes and when they are cleaned up I’ll try them out. I’m hoping to smooth the boards to within sanding tolerances.

Thanks again
BJ

I would use my stroke sander but don’t mind me I’m just blowing my horn because I have one. :-)

-- A tube of calk and a gallon of paint will make a carpenter what he ain't