Readying Metal for the Portable Welder

Simply because you are using a portable welder to do a welding job does not mean that the step of preparing the metal for welding can be bypassed. The accumulation of paint, grease, oil, factory scale, and other substances on the surface of a workpiece will always affect welding negatively, with any results from slight flaws to complete failure of the weld as soon as pressure is applied to it (or even when it is made). Therefore, every effort should be made to clean the metal properly before welding, even if part of the base metal is already fixed in place as part of a construction or machine.

The first stage is to sand or grind both metal surfaces which are to be joined with an 80 grit disc until the metal looks as shiny and clean as possible. Ferrous metals are then etched with degreaser, acetone, or phosphoric acid metal prep to remove the last traces of oxidation from their surface matrix, as well as cleaning away any dust from the sanding. A wire-bristled brush can be used for this scrubbing, but should only be used for specific types of metal so that other types are not contaminated with flecks of an incompatible metal type.

Aluminum is a special case because it forms a skin of invisibly thin but extremely heat resistant aluminum oxide within 8 hours of the skin’s removal. This skin must be removed or its heat reflective qualities will cause deplorable problems when welding is attempted. The proper method is to use acetone and a stainless steel brush to thoroughly scrub the weld area, followed by a heavy rinse of water. The weld must be made sooner than 8 hours or the aluminum oxide skin will have returned and it will be necessary to scrape it off yet again.

An important detail that must not be overlooked when getting metal ready for welding with an arc welder is that a spot large enough to accommodate the welder’s grounding clamp must also be cleaned thoroughly. Most grounding clamps are magnetic, and will not adhere to the metal properly if there is a layer of paint, grease, or oxidation – or simply dirt – on the outside of the workpiece.

Even if the grounding clamp adheres through this coating, the creation of a proper circuit through the metal may be interrupted or modified by the additional material between clamp and surface, causing the welding arc to function unpredictably and thus perhaps produce an inferior weld.

In the event that the paint and other debris coating the metal cannot be fully removed – or if the weld needs to be done in such haste that proper preparation is impossible, an unlikely but not unheard of event – then special welding rod for dirty metal can be used, which will penetrate grease, paint, and rust to produce a solid weld. Several brands of these rods are available on the modern market, and those who might be using their portable welder under less than ideal conditions should probably keep a few of these rods on hand just in case.
Cast metals – specifically, iron and aluminum – must be prepared by preheating, to forestall splitting. A gas torch with a rosebud tip should be used for this purpose, and a heat of 400˚ to 500˚ should be attained.