"the website doesn't say how it will take on Douglas Fir."

No they don't but Old Masters and every other brand name advises testing on hidden spots or wood scraps to see how it looks before doing the intire project . Wood is not a manufactured product and every tree is different.

Regarding prep work,it is thoughly covered on the container as well as website.

It will be worth your time to do some research on staining and finishing, especially fir.

For instance, an oil based finish---like varnish will create an amber tint---very often just what people want.

Woods like fir and pine often need a preconditioner to even out stain color. Stains or dyes can be thinned to change the deepness of the color.

Often testing is the only way to find the right stain/dye/finish combination. Buying some similar lumber and doing the full range testing means the final process you choose will be worth looking at for years.

I built myself a very nice doug. fir screen/storm door. I stained it a fairly light color, and used an exterior poly varnish, but after a couple years the natural darkening of the wood has made it the typical dark golden/orange fir color. I don't think there's any way to tame the natural color of that wood. I don't mind it, even though I'm not crazy about it. It's not a sun-darkening process, either, this door faces north and sees a bit of the setting sun just a few times a year.
Casey

Lisa, I am thinking of getting Fir doors for interior. Are you still pleased with the stain you chose? Can you post pictures?

@lrmcdonald50 we are thinking of doug fir interior doors in light color and are curious to know if you succeeded in your staining? Could you post pictures/stains?

Please post photos of the doors you both mentioned here.