At least you didn't screw up a really nice 24x24 glueup of Cherry - both sides - faced twice on one side - like I DID.
I seal with rattle can shellac - 2-3 coats, sand last coat with 220 - let is dry overnight. Never peeled off any of the finish.
What brand are you using?
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Brent - MDF will not make it outdoors. There is a product called Extira. It is an exterior grade of MDF. Far more heavy than MDF and about 4-5 times more expensive. I am pretty sure I paid a little over $100 for a 1" thick sheet. It took two people to move it. I could not pick it up, I had to drag it. I made a sign component in 2013 and it still looks like new. I have never used MDF outside before, but I DO believe and listen to the pro signmakers on the 3D Sign Forum. I do have experience with Extira. There is another product called Medex - but contrary to popular belief it is NOT an exterior product. Medex is for interior high moisture environments like bathrooms.
This is Extira. This Anchor is 23 inches tall, 23 inches wide and 2 inches thick - made in 2 halves and glues together with Titebond III. I gave it 2-3 coats of Jay Cookes Sign Primer, 2-3 coats of Ben Moore Black exterior latex/acrylic. In this picture it is not finish painted.
Sorry to hijack, but Brent started it.