Pretreatment for coating of Dalian shot blasting

Dalian shot blasting serves as a pretreatment process before coating. Its core purpose is to remove impurities on the substrate surface and form a surface profile suitable for coating adhesion to reduce later peeling and shedding. The whole process shall control cleanliness, roughness and process interval considering the local high humidity and coastal salt fog environment, which is the key of coating supporting services in Dalian shot blasting.

In the early pretreatment stage, workpiece inspection and preliminary cleaning shall be carried out first. After workpieces arrive, check surface oil contamination, heavy rust, old coatings and welding slag. For heavy oil contamination, degreasing cleaning and drying shall be prioritized. Oil entering the blasting chamber will contaminate abrasives and form an oil film on steel, which directly blocks the bonding between paint and substrate. For dead zones such as welds, cutting grooves, deep holes and gaps, bulk welding slag shall be removed in advance by grinding tools, as these areas cannot be fully impacted by shot blasting and are hard to clean only by blasting. At this stage, Dalian shot blasting distinguishes workpiece materials; different abrasives are selected for carbon steel, stainless steel and aluminum alloy to avoid cross-contamination from mixed processing and affect subsequent coating results.

Determine rust removal grade and roughness indicators according to the coating scheme. Sa2.5 grade is mostly required for conventional paint and powder coating. Higher grades are adopted for heavy-duty anti-corrosion systems, zinc-rich coatings or components used offshore. Roughness shall match the film thickness. If the coating film is thin, excessive roughness shall be avoided to prevent exposure at peaks; roughness can be increased appropriately for thick anti-corrosion coatings. Dalian shot blasting selects shot specifications, adjusts the ratio of steel shot and steel grit and blasting parameters including blasting time and conveying speed based on customers’ paint instructions, to achieve uniform surface profile without over-blasting or insufficient cleaning on local areas. Blasting angle shall be adjusted for complex special-shaped workpieces to ensure shot impact on inner cavities and back sides.

Control equipment and environment during blasting. Keep stable operation of the dust removal system inside the blasting chamber. Accumulated dust reduces cleaning effect and interferes with coating if attached to the surface. Abrasives shall be screened regularly to remove broken fine shots and impurities, which cause disordered surface texture and fluctuating roughness. Humidity is relatively high in Dalian. When humidity is high and condensation easily occurs on workpieces, suspend blasting or preheat workpieces to keep their temperature above dew point and prevent rapid rerusting after blasting. Separate blasting and coating zones at the workshop to prevent dust drifting into coating stations and contaminating paint films.

Conduct rapid dust cleaning and inspection after blasting and control the coating interval. After discharge, blow off residual shot fragments and dust in gaps and pits with brushes and clean high-pressure compressed air. Inspect the cleaning grade against reference samples and spot-check surface profile with roughness testers. Unqualified workpieces shall be reprocessed. In offshore environments, shorten the interval between blasting and coating. Primer application shall be finished within two hours under high humidity. Once flash rust appears, re-blasting is required instead of direct covering by painting. Dalian shot blasting informs customers of the coating time limit together with delivered workpieces to control the connection schedule.

Implement workpiece protection and record-keeping for different delivery scenarios. If workpieces are transported to other factories for coating after blasting, cover them during transit and avoid outdoor stacking to prevent contamination by salt, rain and dust. Keep transport brackets and packaging materials clean to avoid secondary pollution. Keep test records in batches for mass workpieces, marking rust removal grade, roughness and processing time for verification in subsequent coating procedures. For workpieces requiring temporary storage, temporary anti-rust measures can be arranged after negotiation, and the anti-rust material shall be compatible with subsequent coatings.

In summary, Dalian shot blasting for coating pretreatment starts with preliminary degreasing and slag removal, sets blasting indicators matching paint requirements, controls the operating environment to reduce condensation and rerusting, thoroughly removes dust after completion and strictly controls the coating interval, so as to provide a stable and qualified substrate surface for subsequent coating.