Anyone know to pretreat raw cottonwood before varnishing it?

Cottonwood is very porous. It will drink up lots of varnish before enough can build up on the surface to create a smooth finish.

You will want to give it a good sanding before starting, starting with 100 grit and ending up at 220 grit. You’ll have to decide whether you want to stain the slabs or leave them natural toned. Some stains also seal the pores so varnish applied after the stain will build up better on the surface rather than sinking in. And frankly, cottonwood isn’t a particularly attractive color without some help.

After staining, sand lightly with 400 grit paper and then apply your varnish. Count on several coats even if you’ve stained and sealed, and still more if you haven’t. Be sure to sand lightly again with 400 grit between each coat.

I’d be tempted to use an oil finish rather than a varnish for cottonwood. I like tung oil, personally, but linseed would also work. That is, if I were using it for a table or a bench or something. Definitely varnish if it’s going to be a countertop, as poplar is very soft wood.

Oh, and don’t even think about applying any kind of finish until those slabs are thoroughly dry. If they’ve just been cut, this can take quite a long time, as in a year or so.

Cottonwood I knew from the masses of “cotton” drifting around the subdivision where I grew up. Poplar is the cheap “hobby wood” boards at the hardware store.

I did not know they are from the same tree. Thank you, @Seek.

A Mennonite farmer I know calls the wood “popple”. Has anyone else heard that before?

Thanks to all for the advice.
The person hired to cut these tress told me that they were very dry to begin with as they grew too close and did not get the energy to grow .( water?)
I do not have tools, so I will have to sand them by hand..it it doesn’t take up too much time.( I live in a apartment and have no where else to do this.
I planned on using it as small end tables ( they are both 17 inches by 15 inches in diameter.
the smaller chunks ( wedges)(4) I plan on using them as bookends. I researched and realized that the “bark” of these trees are valued due to their scalloped designs. This shows predominantly when placed on their sides. Two however where in the rain ,but surface is dry now. However insides may not be. So will not varnish as advised by @thorninmud says.
This is the first time I will try this hobby, I like wood.

@Seek
Thank You also for your advice too.
I heard that spirits where to be used to clean it? ( not sure what that means.?)

Congrats to @thorninmud for reaching 20K !

Methylated spirits is just British-speak for denatured alcohol. You can get it at Home Depot or Lowe’s.

Sometimes when I’m slumming it I just use 70% alcohol from the drugstore. Denatured alcohol is 90% alcohol.

@Seek
Ok Thank You

It has only been one day and the cottonwood disks are cracking!
The stores are closed today as it is a holiday…any advice to help this situation from getting worse?

Phew!
located a wood site that had an old blog forum on it that adviced to keepcotton wood from drying and cracking too soon, too warap the raw slices of wood into wet towels..of which I have done and placed into plastic bags until I can get it too the lumber store to get pretreated if possible to prevent fast drying and therefore cracks to appear.
found another site that recommends Pentacryl wwod preservative treatment first, then dry then sand in time.

I have located an artilce that outlines procedures to preserve raw tree slices and more.
It is a free pdf version ( 1972..so I hope procedures are still advisable?)

“How PEG helps the hobbyist who works with wood” By H.L. Mitchell

In case anyone wishes to learn on how best to preserve wood.

Honestly, I’d trust a woodworking guide from ‘72 more than I would trust one made today. Good find.

@Seek
Thank You
This simple idea has become a much more project then anticipated.
I now have much more respect for the wood carvers and preservers of the world.
It is painstaking work to come up with a beautiful one time product.