I found myself at the beginning of 2015 to be in need of a bigger desk then what I had.  The desk I had was only four feet long and 18 inches wide.  Not much could be done on that desk.  I was constantly stacking things up around the desk and on chairs or throwing stuff on my son's desk.  When I bought my house the old owners left quite a few odd and ends about the property and in the spare garage.  One of the things they left were 2 old file cabinets.  They continued to sit in the garage collecting black widows for several years.  Now that I was in need of a bigger desk and no money to buy the one I wanted, it was time to be ingenious.  I had been at one of the warehouse stores the week before and had them cut down a birch plywood sheet for some shelves.  I had a piece left over that was 62 inches by 24 inches.  It was not quite deep enough because the cabinets are 26 inches in depth but I thought, what the heck who would really care.  I already had the birch trim to go around it so the only thing left to buy was the spray paint for the cabinets.  
I used a spray paint for metal that also contained a primer. The white took me 4 cans to cover the two file cabinets.
I started out by cleaning the cabinets, removed a few spots of rust with some steel wool.  The spray paint that I purchased already had the primer in it.  I used spray on the metal so that I didn't have the brush strokes.  Just remember when spray painting not to rush it.  Use overlapping sweeping motions back and forth.  Apply a light coat, let it dry and then another and another until at the thickness you like.  Make sure not to over spray or it will drip.  Let it dry between coats.
The Red spray paint also contained a primer
For the top, I sanded the wood first with a 60 grit followed by a 100 then a 200.  Wipe it down with a lightly damp cloth to remove all sanding dust.
Let the wood dry.
I used a nail gun and glue to apply the trim just on 3 sides (not on the back as the wood was not wide enough. If you wood is wide enough then go ahead and put trim on the back.  I sanded the trim before attaching.  
Fill nail holes with wood filler, wipe and let dry
Apply the stain that you like.  I used a Golden Oak. Apply in the direction of the grain.  I use a lint free towel to apply and a lint free towel to remove excess.  Go in the direction of the grain.
Let it dry for 24 hours.
I then applied my polyurethane (use a good brush so that it doesn't leave stroke marks)
I applied one coat.
Let it dry.
Sand with a 200 grit.
Use a lightly damp cloth and wipe off dust.
Let dry 30 minutes
Apply a second coat of Polyurethane.
Repeat above.
Apply a 3rd coat and repeat above.
That's all it took.  Seriously.

Total cost was about $30 and that includes the wood piece used (originally $44 for the entire sheet), polyurethane, stain used, sandpaper, spray paint (I used 4 cans, 3 white and 1 red), steel wool.  I did see some of these cabinets at a garage sell for $4 each and over at the thrift store for $5 each.  You could also use a pine plywood sheet to reduce the cost.  The most expensive was the spray paint and I could have gotten it cheaper with coupons or at a big box store.

I hope that description was not too confusing!  If you have any questions, please email me or leave a comment.   I also have a YouTube channel where I feature many of my recipes and Challenges and soon some of my DIY's.  If you would like to check it out, click on this link:  Bristlee One

Thank you for stopping by and remember, enjoy life.
It's the one you have,