So I started welding today on my build, lots of practice first, then some "real" welds.

(Using a MIG with flux core wire)

Some of my welds don't look so pretty, but I did get good penetration so I believe they will hold ok.

My question is this: if I grind down the not so pretty top side even to the metal, given good penetration, does that affect the weld strength or should I leave well enough alone?

Coming from a certified welder,(not boasting just helping) If you did indeed get good penetration you can dress up the welds and be ok. When you start grinding, watch for hair line cracks in the base of the weld. If you see these, grind it down real good and run another bead over it. Repeat as necessary.

Load up all your welders Jody, and head down I-24. I got a few things I need you to do.

Jody is right on. Most of the time you get an ugly weld from bad ground, dirty metal, and gun position. Well speed to I quess. It's the position of the gun alot though. Grind it down and see. Look at the back side of the weld to see penatration. Over all a weld does not have to be pretty to be strong

I call them gorilla welds,uglier than dirty diaper,stronger than he77

As long as it's strong

I ground a few of them down a bit today and they looked fine, so I decided to leave ugly enough alone.

On the bright side, today's welds both look and are much better.

Thanks guys.

yup that flux core is dirty but clean her up with a grinder and hit it again 25 year certified welder here if i can be of any assistance just ask

If you've got a gas option on your welder use it..
It produces a much better weld.

My son (ACBliind) started welding to fix our buggies.
He was able to get progress well with the flux core wire.
With the gas the improvement was incredible.