From AWS D1.1/D1.1M Sections 7.3.1.4 and 7.3.1.5, “Welding consumables that have been removed from the original package shall be protected and stored so that the welding properties are not affected. Electrodes shall be dry and in suitable condition for use.” In addition, AWS D1.1/D1.1M Section 7.3.2 contain provisions for storage and baking for low-hydrogen electrodes, which are more susceptible to moisture absorption. FCAW electrode to be used on a seismic load resisting systems cannot be exposed more than 72 hours unless a longer exposure time has been proven by test.

ASTM A36 up to ¾ in. thick steel is classified as AWS Group I material and, as such, may be welded with non-low-hydrogen processes. In contrast, ASTM A572 Grade 50 steel is classified as AWS Group II material, which, because of its higher yield strength, must be welded using low-hydrogen processes. Because dual-certified steel, by definition, meets the chemistry and strength requirements of ASTM A572 Grade 50 steel, welding should be performed using low-hydrogen processes, unless the suitability of an appropriate weld procedure specification using a non-low-hydrogen process can be demonstrated through qualification testing.

Seal welds are sometimes made to provide a water- or air-tight joint that otherwise would not be. In building construction, seal welded joints are rarely required to withstand internal pressures as would be common in steel tanks and piping circuits. Consequently, they can be sized for any load transfer requirements or from minimum size requirements in AWS D1.1. Welds smaller than AWS minimums may be desirable. Heat input, relative movement of the connected elements, and pressure should be considered when making this choice. 

An alternative to seal welds is to caulk with a clear silicone to keep water out.

In most case, seal welds commonly assume a fillet weld profile. Any aesthetic requirements for seal welds should be specified in the contract documents.

Possibly. If the chemical properties of steel to be welded are known, either by valid mill certification or by laboratory sample testing, its weldability can be judged by computing the carbon equivalent value. A more obvious approach would be to examine the existing structure for evidence of original welding. Alternatively, an on-site investigation could be performed to address weld ductility and base-metal hardening. Other factors should also be considered, such as past history of the structure, the nature of the loads, weather conditions, and whether the members to receive welds are loaded; refer to Ricker^[1]. Guidance related to welding involved in seismic retrofits and rehabilitation is provided in Seismic Provisions for Evaluation and  Retrofit of Existing Structural Steel Buildings ANSI/AISC 342. Guidance on evaluation of existing structures and weldability is also in AWS D1.7 Guide for Strengthening and Repair of Existing Structures, and in Section 14.9 in AISC Design Guide 21 Welded Connections-A Primer for Engineers Section; refer to Miller^[2].  

^[1] Ricker, D.T., 1988, "Field Welding to Existing Structures," Engineering Journal, Vol. 30, No. 1, (1st Qtr.), pp. 44-55, AISC, Chicago, IL.

^[2] Miller, D.K., 2017, “Welded Connections – A Primer for Engineers, Design Guide 21, AISC, Chicago, Il.

Weld metals are matched to the steel grade being welded. Matching weld metals are specified in AWS D1.1/D1.1M Table 5.4.