Even if you are able to get behind the panel with the holes and seal it, it is still not wise or cost effective to shortcut the repair if you have the means to weld the holes up. If you decide to braze instead of weld, be sure you get ALL the flux residue from the weld area as it will grow hair very similar to you favorite bread mold...Bondo is very much like brake fluid as it will absorb moisture from the air as the panel heats and cools so if moisture is able to condense..well..it sucks it in..then when the sun hits the next day..it will draw the moisture right on through and blister your top coat...I believe everyone has seen examples of this.