Wanted: help with reflow oven

It looks to me as though that board was never meant to be reflowed in an oven. Instead, it was soldered using something called the Wave process, where the board is literally passed over a “wave” of molten solder. Anywhere there’s a hole is filled with solder.

One easy way to tell is all the through-hole and plastic components, the single-sided board, and the lack of surface mount parts. Next time, use a soldering iron and a desoldering pump – unless you happen to have access to a solder fountain!